The company says customers squeezing themselves into a KK-cup — a size launched two years ago to meet increasing demand — have been campaigning for larger bras ever since.
And although an L-cup remains far larger than the national average, there can be no doubt that when a High Street chain sees reason to supply it, demand is growing — and so, it would seem, are our breasts.
For women such as 21-year-old Terri Smith, from Stourbridge, West Midlands, who wears an uncomfortable KK-cup, the L-cup could provide welcome relief from the daily agony that comes from forcing herself into ill-fitting underwear.
'Every woman knows how uncomfortable the wrong size bra can be,' she says. 'The wires dig in, the straps leave angry red marks on your ribs and your shoulders, and your breasts spill over the top. None of your clothes fit properly. But when your breasts are this big and heavy, it's unbearable.'
L-cup breasts weigh approximately a stone. Since Terri was a teenager, she has sought out specialist lingerie and suffered almost constant physical discomfort as a result of carrying such a heavy load.
As a size 10/12 everywhere else, she believes her true bra size is a 30L, but until now has struggled to find them.
According to Bravissimo, there are many more women of Terri's extraordinary dimensions out there. But it says this unprecedented demand for the L-cup can be explained, in part, by the fact that for years women have worn bras that were too big in the back and too small in the cup.
As more of us grasp the idea that a larger cup size and a less generous back-measurement could give us greater comfort and better shape beneath our clothes, we are buying bigger sizes than ever before.
But, according to Bravissimo's estimates, at least 60 per cent of women wearing a C-cup should be wearing a D-cup — or larger. And it says the average British bra size is closer to a 34E than the 36C we are led to believe.
But we don't owe our expanding bustlines to modern fitting techniques alone.
Back in the Fifties, the average woman wore a B-cup — a full size smaller than today's larger size. It's clear that our dimensions have shifted, but why?
Fat is the first answer most experts will give. Professor Michael Baum, an expert in breast cancer, says: 'Fat is laid down on breasts as much as thighs or bottoms. We are experiencing an obesity epidemic, so the increase in women's measurements isn't that surprising.'
But this is only part of the story. After all, women such as Terri do not appear to be carrying much excess fat elsewhere on their bodies. As Terri says: 'The rest of my body is quite slim. Yet throughout my teens my boobs went up a couple of cup sizes every year.'
Anna Prince, from Bravissimo, agrees: 'There is a total misconception that it's unusual to be big-boobed and small-bodied. We've been contacted by more than two million women since we started in 1995, the vast majority of whom are small in the body and big in the bust.'
So, what's going on? Why are the nation's breasts getting bigger?
Dr Marilyn Glenville, a nutritionist specialising in women's health and hormones, says: 'It's clear that we're not just talking about fat, but increased levels of breast tissue, too.
The link between increased oestrogen levels and bigger breasts is so clear that there are even 'breast-enhancing' supplements on the market — such as Perfect C Breast Enhancer capsules — containing ingredients such as fennel seed and fenugreek, which are said to have oestrogenic properties.
Dr Glenville says: 'It makes sense to look at the ways in which our exposure to all types of oestrogen — the hormone our own bodies produce and oestrogenic chemicals we come into contact with — has changed over the years.'
'Girls today reach puberty earlier than ever before, and are going on to have fewer children and breastfeeding for less time. As a result, we have far more periods than our ancestors would have had and we are exposed to more monthly surges of oestrogen, which stimulates ovulation.'
So, could the changing shape of our breasts indicate an increased sensitivity to oestrogen?
Dr Glenville says: 'Pregnancy and breastfeeding have a protective effect against breast cancer because they control the hormones which stimulate the growth of new cells in breasts.
'But with more women today putting off pregnancy until later in life and having fewer children, they experience many more monthly cycles than previous generations did, and are exposed to more oestrogen.
I'm certain that if you looked at photographs of Victorian women, who on average had five or six children, you'd find them comparatively flat-chested.'
But, of course, that is far from the only difference between women's lives then and now.
HRT also tops up depleting oestrogen levels in menopausal women, who — like women on the Pill — often go up a cup-size or two when they begin a course of treatment.
But it's not just women on the Pill or HRT whose oestrogen levels, and cup-size, might have increased as a result.
In 2002, research published by the Environment Agency showed that an 'exquisitely potent' form of oestrogen — which is believed to have entered the rivers through the urine of Pill and HRT-users — was responsible for changing the sex of half of all the male fish in British lowland rivers, and could be contaminating the water supply.
Now, it has been suggested that the influence of these xenoestrogens (literally 'foreign oestrogens') could be responsible for the rapid decline in male sperm count and fertility.
'We can't assume these pollutants have no effect on us,' says Dr Glenville. 'There are many questions still to be answered, but if xenoestrogens are potentially responsible for declining male fertility, they are potentially affecting women, too — and the proof could be in our bras.'
So how do we avoid these surplus hormones? The answer is, we can't. And it may come as a surprise to know that they are found in everyday items.
'Pesticides, plastics and cosmetics are my main concerns,' warns Dr Glenville.
For instance, a xenoestrogen called Bisphenol A (or BPA) is widely used in the manufacture of tinned food, drinks cans, plastic bottles, glass jars, electronic equipment and till receipts — to name but a few items.
Although the European Food Safety Agency maintains that BPA doesn't pose a risk to the public, many scientists consider it to be a potentially harmful 'hormone disruptor', and several of the world's leading food manufacturers are putting timetables in place to remove it from all of their products. Heinz insists it's at 'an advanced stage' of removing the chemical from its UK baby food range.
But, until now, our exposure to it has gone virtually unchecked.
'The same goes for xenoestrogens in the deodorants, make-up and moisturisers we use,' says Dr Glenville.
'We apply them to our skin and often directly on to the breast. Our skin absorbs those chemicals readily. It is not inconceivable that those chemicals stimulate growth in breast tissue.'
But we're not just covering ourselves with oestrogen, we're drinking more of it, too.
The introduction of intensive dairy farming methods to maximise production means that about two-thirds of the milk we consume comes from pregnant cows. To ensure that a dairy cow has a steady supply of milk, she is almost constantly pregnant.
But taking milk from a pregnant cow, especially during the last few weeks of her pregnancy, raises questions about the high levels of oestrogen and other hormones in milk — and how they might affect those who consume milk every day.
'It's not just about exposure to oestrogens, but how our bodies cope with them,' says Dr Glenville. 'It's possible that increased alcohol intake impairs the liver's ability to help us metabolise and excrete excess hormones.
'We also live more sedentary lifestyles these days, which may mean we metabolise these hormones less quickly.
'Hormones that aren't efficiently excreted can re-circulate in the body and the cumulative effect of this may be a build-up of oestrogens, which — over a long period — could alter our natural body shape. It's something we should take notice of.
'After all, developing very large breasts can have all sorts of health and wellbeing implications — and forces a lot of women to consider breast-reduction surgery.'
Big breasts can affect posture, causing chronic back pain and leaving permanent indentations on shoulders where bra straps cut in.
Health problems can be emotional as well as physical — some women are left very self-conscious by their large chest and the undue attention this attracts, which is why they have turned to professionals such as Dr Puneet Gupta of The Private Clinic of Harley Street.
Due to an increase in women inquiring about breast reduction operations — each year around 10,000 women pay up to £5,000 for private operations — he is pioneering a new kind of reduction surgery called Microlipo, which reduces the risk of breast damage associated with older surgical techniques.
'Like all forms of surgery, breast reduction is now more widely known about and more affordable than it was,' he says.
'There have always been women who have disproportionately large breasts, compared to their frame. But they are more likely to seek help now. The women I see are usually sick of the physical and psychological discomfort.'
Many other women, however, such as Terri Smith, learn to adapt their lives rather than go under the knife. Terri was forced to give up her job as a hairdresser because her breasts gave her such terrible backache.
And Claire Smedley, a 28-year-old model and mum-of-three from Derbyshire, can think of several ways in which her L-cup breasts have impacted her life and her health.
'Although I've learned to love them in recent years, I resented them for a long time,' she says. 'As a teenager I didn't have the confidence to carry them off, and I was constantly dealing with unwanted attention from men.
'Even now, because my breasts are so large, strangers will grab and touch me when I'm out, sometimes in front of my children.
'They think I've put them on display — that I'm asking for it — when, of course, there's little I can do to hide them.
Women are wary of me, because they see my figure as overtly, perhaps threateningly, sexual. I'm automatically considered a tart.'
When Claire was pregnant, and her chest ballooned even more, the weight of her bump and breasts combined put her body under enormous strain, causing heart palpitations.
'It's not as if it's easy to stay fit when you're this size,' she says. 'Any form of aerobic exercise is agony and sports bras are hard to find.
'I've thought about breast reduction, but the fear of something going wrong stops me going ahead. As I get older, if I suffer from back pain I will reconsider, though.'
So until scientists unravel the mysteries of our growing assets, more and more women will still suffer the pinch of underwear that just doesn't come up to (ample) size.