About breastfeeding, is it really true that women with small (30aa) sizes can make enough milk, or do doctors just say that so women won't feel badly?

Size doesn't matter: Your breasts will make as much milk as your baby needs. Your breast size does not affect that. Your breasts may slightly get larger during breastfeeding but it does usually go back down to your regular cup size when you stop breastfeeding.

Yes: I recall a perfect example of this in my practice where the mom had pleanty of milk. She actually succesfully fed two kids that thrived, she had so much extra she donated to al banked breast milk program.