I'm a 31 y/o 32 B, but I assure you I have super tiny breasts I can fit in some 32 A bras. I was body shamed all the time when I was younger, but truth is I look better than most women much younger than me.
It took years before accepted my body as it was and didn't feel the need to wear padded bras or anything. You're making it sound like her breast size is negative, and you're probably acting out that feeling towards her. I've dated men with small penises and I just think I wouldn't change him. My first serious bf at age 20 always made remarks like "you'd be perfect with big boobs" and my 1st boyfriend offered to get me implants. I quickly dumped those losers.
If you prefer a larger chest go meet a woman who has one, otherwise accept her for who she is. Also your friends sound like idiots, a real man would never allow his buds to trash talk his girl. Tell them that those breasts are for you, not for them and they can shut the h up. Also how many of these jerks even have a girlfriend? Body shaming is so bad, did you know women who get implants have significantly higher suicide rates? Women with small breasts attract men with higher incomes, tend to be thinner and are less likely to date chauvinists so yay for me! Google the studies it's all true. Men who prefer larger breasts tend to be less educated, hungry, poorer, but women with larger breasts have about 2 higher iq points on average than smaller women. I think your girlfriend can do better and if you're that upset about her body then dump her. Maybe one day you'll be more mature. Your buddies sound like chauvinists btw. Good luck