Breasts are very visually pleasing to guys especially big ones. They're a big thing, they're important, and, sadly, girls are handicapped for not having big ones.
However being fat with big breasts (and breasts are fat, so fatter you get bigger they get usually) means you're fat.
I would far far rather a slim girl with a nice physique with smaller breasts to a fat girl with big ones.
In fact, I doubt I'd ever sleep with a girl who was overweight. Shallow, maybe.
In practice, it doesn't matter. Because guys do look at your face and your physique above all else - if breasts are covered and they're not big, it's a guessing game how big they really are. By the time you come to sleep with a girl, you don't really know how they'll look naked, you can guess - and by the time a guy sees your breasts he wants to sleep with you no matter their size.
I've slept with a girl who didn't wear low cut tops, after sleeping with her thought her breasts were small - she was a D. Slept with another girl, thought she had big breasts, she was a C. One girl had really nice ones and she was an A. Men's perceptions can be influenced by your behaviour, your dress sense etc.
Maybe girls with big breasts get more attention if they're on show - it makes them look more confident sexually, more likely to be easy and put out, maybe because it's more socially acceptable to glare and it be funny - and attract their attention and let the girl know they're attracted to her without it being creepy.