Selection pressure predominantly comes from the female. Females "look pregnant" e.g. boobs (name another animal that has boobs while not pregnant!) Men in turn find boobs appealing. But the intention was to make women seem unavailable. Boobs don't look good, in the same way sugar isn't sweet to animals that can't process it (cats). In other words, I like boobs.
Discussion
Yes, selection pressure comes from the female, but women tend to share males. That's why humans have significantly fewer male than female ancestors.
Males with high status therefore often have/had offspring with multiple women. Some even with hundreds or thousands of women. Those men were each their own evolutionary bottleneck, so we're a reflection of their choices.
Also, full breasts combined with a low WHR, is a clear fertility signal and those women are known to have more offspring. I suspect men eventually developed an instinctive knowledge of that true signal and learned to prefer women who displayed it.
Women who "look pregnant" -- even before they are pregnant -- are signaling that they are more likely to become pregnant.
This makes more sense, if you look at the historically high rates of infertility, infant starvation, and maternal death during childbirth.
A womans' ability to get pregnant, deliver safely, and breastfeed a child during lean years was often the difference between a man having progeny and not having it. We are the descendants of those men.
I therefore see the anti-breast movement (because that's what it is) as a part of the larger anti-natal movement that is driving the transgender beauty ideal.
It's all part of the insididous Culture of Death.
Women with full breasts tend to be mothers or women who will become mothers.
Mothers are bad. Throw rocks at them.