LogFAQs > #979894222

"There are two reasons for this," explained Shuko Sakata, manager of brand marketing at Triumph. "The first is changes in the diet, such as increased meat consumption and westernization in general. The other is because we manufacturers have become better at teaching customers the correct way to select a brassiere. When putting on their bras, women tend to lean forward and by so doing gravity collects fleshy parts on the sides of their torso to fill up the cup. That alone can increase cup size by as much as two sizes."

So it sounds like those that are getting bigger are due to diet, and otherwise it's just a change in how they actually select bras, not actually necessary really being bigger.