It is true that bust sizes are getting larger. But the reasons vary and complex for every woman.
First off, I have a friend who is a highly sought and well known designer in the fashion industy. They have told me many times that the sizes have been changed over the last 25 years. Twice changed for some lines and designers. What was once an 6 is now a 4, an 8 is now a 6, a 10 is now a 8, etc, etc. Also, sizes were created to get the market adjusted to actual women. A size 20 and 22 never existed a dozen years ago. Neither did the size 00. The size 00 was created for the new super thin and the older size 0s which were rare themselves. These changes are not recent but occurred over 20 years ago in the late 80s and have been ongoing. A woman who who wore a size 6 years ago is now an old size 8 but she still buys a size 6 and is happy. And a woman who wore a size 12 in college now still wears a size 12, but the actual size would have been a 14 years ago. She is happy even though she has gotten bigger.
Bra sizes are different. They have been changed little, but larger sizes are created.
NO TWO BRA COMPANIES USE THE SAME SIZE SCALE. That is the first problem in bra size.
A Goddess Brand 36H is not a Fantasie Brand 36H and certainly not other makers as well. The cup sizes vary greatly in the lettering.
Some brands skip some letters in the alphabet and some double the letters (e.g. DD, F, G, GG, I and another brand is D, E, F, G, H, HH, J, JJ)
No wonder an estimated 95% of all women wear the wrong size bra. Band size and cup size wrong! Women tend to buy the same size and then stick with it and not realize their bodies change. It doesn't fit well but they put up with the pinching, pulling, overflowing, underfilled, and lose and too tight straps and more.
Also, for many women their hormones change in life. Weight gain for some women the weight goes all over. Some women only in the thighs and/or butt and/or stomach. Some in their breasts. (Lucky girls to my view...lol).
There is some anecdotal evidence and common belief that the hormones in the dairy products we have consumed, and other food additives have had a cumlative effect on our bodies. Higher incidence of cancers and other diseases, increased weight gains, coronary disease, and a host of other things. One thing that is often thought is a result of the additives is larger breast growth in both women and some men. (Man Boobs). Because all the chemicals and diets of everyone is different, and it all reacts with the varied lifestyles and individual genetics, the results are so varied it is difficult to say for sure. People are also healthier in many regards and grow taller and such and while it may be that a side effect of the better diets for male youth is larger adult genitalia, it may also be a reason many women seem to be having larger female genitalia and also larger feminine busts.
The science is still out on that and all is speculative for any one person.
Also, I know that some women just "blossum" later in life for no other reason than "hormones". I know a woman who was a D cup all of her life. In her early 40's she suddenly started to grow bigger breasts. 36JJ now. She has no idea why or how and she has never had children. She is in her early 50's now. She has finished menopause a few years ago. Other than being one of the horniest women alive (she is candid and describes herself as a true nymphomanic), she grew a huge bustline. She thinks, but can not be sure, it all came about by her hormones going nuts in her early 40's and she had a normal bust and a normal sex drive before that. Now she is insatiable and a huge bust. Honest truth.
So hormones and chemicals/food additives affect us all. Some in small ways and some in larger ways.
But the fact that women are getting larger busts than the earlier generations is true and part is diet, part is additives, part is just much better health in general, and part is hormones, and some is no doubt genetics.
