Unless he is putting you down and making you feel bad intentionally about your smaller boobs, or hurting you by saying he wishes you had bigger boobs, you have absolutely nothing to worry about or be upset about.
Take this from someone in the same position. My boyfriend is fantastic, is affectionate, tells me I'm beautiful, etc. He is dedicated and wants to be with me... But guess what? He likes big boobed, in shape blondes. I'm a small boobed girl with black hair and enough tummy for a person to reach out and get a handful. And it isn't as though he couldn't get his 'physical ideal' because he is a great guy, very attractive, in shape, etc. Despite that I'm basically the opposite of his 'ideal' doesn't change that he is with me for a reason, and part of it (though it isn't the only reason, of course) has to be that he thinks I'm attractive since this is required for sexual chemistry.
So the same applies for you. Your boyfriend is with you for a reason. Just because big boobs is part of his ideal perfect woman fantasy, doesn't mean he cares about you any less or is any less attracted to you. He wouldn't be your boyfriend if he didn't like both your personality and appearance. Think about your boyfriend? Is he 100% everything you like physically? Even if you think he is attractive, it is unlikely that he perfectly matches the physical ideal you have in your head of what your perfect man would look like. Be honest here. So he is in the same position as you!
Oh, and in regards to him looking at other girls... All guys look at girls, whether they are in a relationship or not. They're human beings; they have eyes and sex drives. They'll stop looking when they're dead. It isn't a bad thing; I'm sure you look and take note of other attractive guys that aren't your boyfriend! there isn't anything wrong with that; its healthy. Now, if he approaches other girls, that is another issue, but there is nothing wrong with looking. Being in a relationship doesn't make every other person in the world ugly. Just relax and let him look!