I dont know if there was ever a time when you may have made him feel bad, or "cracked jokes" regarding his looks, or weight; but it seems that he was once insecure himself and now that he's worked on his appearance, he's just feeling like he's got his mojo back.
I dont think that you should leave him because of this, instead I think that you should work on yourself, and put yourself in the same position that he's in. I'm not saying loose weight and go hit on men, but try anything that will give you some inner peace. Even if it's changing your hair, or buying some new clothes. Maybe you should get a babysitter for you children( if they're young enough) and go hang out with friends. Give him a little taste of the fact that your getting your mojo back too.
Sometimes when we are in relationships, we get comfortable in thinking that we dont have to keep ourselves up, or that the underdog of the relationship will always stay the underdog of the relationship. Sometimes we feel as if the one that we are with will always love us irregardless of our changes in life. That is true to a certain degree, but you have to constantly keep stepping your game up, not for competition because there will always be someone prettier, richer, smarter, thinner, or more sucessful. You just have to do it so that you know within yourself that you did everything you were supposed to do, not only for him to love you, but stay IN LOVE WITH YOU. and if you two have just gotten tired of one another, you can always look back and say at least I conditioned myself for the next relationship ( I dont see that happening with you two though).
Love Honey