>>1571738um, because when girls compare breasts, it's the guys that WIN!
Seriously though, it might be partially because we'd like to "console" the smaller breasted girl and have a chance to show her she's sexy too. Or maybe, like me, some guys like ALL sizes of breasts and so you get to see sexy little boobs along side sexy bigger boobs. How can you lose if you like them both? You can't!
