guys like tits because it's part of our biological construct
larger breasts mean more fertility for their children
same why girls like successful/rich guys
Larger breasts have no impact on fertility or milk production. Evolution works by there being a benefit to random choices. Our ancestors that chose large-breasted women wouldn't have produced more children than those who picked small-breasted women.
It's not a biological construct, but a social one. Big breasts are appealing because they look like wide hips and thighs, and those ARE indicative of child-bearing capacity.
One theory I heard was that it pertains to humans having sex facing one another when we evolved to be more bipedal, and larger breasts indicated a figure looking more feminine. Before that we were all about the booty.