I found this thread so, so sad. People have been very quick to judge this man, in a way they wouldn't have if his appearance had changed for an accident or something similar, this is because overweight people are judged harshly in our society, obesity is seen as a character flaw, a moral failure.
We know today, if you bother to look at the science, that this is not the case, there are a number of complex factors that explain it, that we do not even fully understand yet.
One just has to look at the size of the dieting industry to realise losing wight and keeping it off for good is very, very hard.
OP, if you don't love him anymore, are not attracted to him anymore, fair enough, of course you have to leave, you have a right to be happy. But if you think he must not love you because it would be so easy to make a change but he won't, I don't think that's true. It is not easy to make the change. All I am saying is, don't take it as a sign he does not care about you in itself. Of course, for all I know he may be a selfish prick nevertheless, only you know that!
Obese people wear the signs of their struggles on their body for all to see, they can't hide it, this makes them really vulnerable, and opens them up to judgement immediately upon meeting, that in itself becomes a burden.