Let me honest (and hope it isn't harsh, it isn't meant to be...)
If a guy tells you your boobs are too small (or too large!) for him, then he's just being a jerk, plain and simple. If the size of your boobs is a dealbreaker for someone, and they can't see or admire your other wonderful qualities, then they're just an a-hole.
Everyone has a personal preference; some guys might like an A cup, other guys might like a G cup. Don't worry about your boobs being too small or too large (they're actually average). There are loads of other characteristics you have that will probably matter way more...and if a guy makes you feel bad about your boob size, he's not worth your time.
I highly doubt you'll ever hear a guy say ''I like you but your boobs are too small''