You have someone that loves you. I'm fairly certain that when he looks at you, he sees the things that you can't see in a mirror. I have been in the underweight category my entire life, and I've always dated bigger guys. Bigger guys are more fun to be around. This thread seems to be all on the same page.
It seems to boil down to this though- you and your partner love each other. Ask him what he sees when he looks at you, don't ask a mirror… mirrors never have anything good to say