To offer a point of view (that might get downvoted), while I'm not as bad as your boyfriend, I act similarly towards my wife from time to time, although I try my best not to.

For me, much of this is a habit I've picked up from my dad. My father was very critical of me and my mother when I was growing up and I learnt that habit myself. In consequence, I'm a very critical person, both about others and myself.

My dad would always criticise my mother's weight and from time to time I find myself having similar feelings about my wife. I'd never call her fatty or anything (although my dad would say that to my mother), but I do worry that she (or I) will "get fat". If she's having some fatty food I'll be a bit critical.

It's hard to get over this type of habit. He may not even realise it's a bad behaviour if he does it automatically.

While I can't categorically say it's the same for your boyfriend, it could be a possibility... If he was overweight when he was younger and his parents or other were critical of his weight, it might have given him a kind of neurosis about it. Making him critical of other peoples weights and possibly even his own.

Something I'll say about critical people is (from my own experience), while you may think they're being a dick, they're often also just as critical of themselves. It's hard to be like that as you're always looking for the things you (and others) do wrong. You can often take little satisfaction from doing things right.

Anyway...none of this excuses the behaviour at all. If your boyfriend truly cares for you, he'll do his best to try and better his behaviour if he sees it hurts you.

But what I do think this does, is it explains that this behaviour really has little to do with you. My own mother's way of dealing with my dad is that she doesn't take him seriously at all. But I'm not sure this really creates a positive relationship as respect is lost...