Why are small "bad"?

I know exactly what you mean. I'm a b-cup but on the smaller side. I've always been self conscious bout them around guys, by I like them the way they are. My current boyfriend used to tell me how much he loved my and it made me feel good. Until the one time he said "well yeah I like them the way they are but sometimes I wish they were bigger" it's like oh thanks. But then if I wear a shirt that show he makes fun of me for havin them "hanging out" when there not. I wish I knew why guys go with the bigger is better stuff to it's annoying have the guy you love check out other girls and say oh she hot when you really don't wana hear it.