In the long term, no one is quite sure what it is that causes humans to have permanent breasts, given that no other primates do. There are theories around fertility signalling, evolutionary selection via mate preference, extra insulation given the lack of body hair in humans, extra fat deposits to get breast feeding folk through feeding a new born in times of scarcity etc, but no one can say for sure…

But none of that has anything to do with why breast size has increased so significantly in the last several decades. Circumstantial evidence links that to increasing body weight, as the most significant increases are in countries with the biggest average increase in body weight.