If he, out of the blue, said, "You're too fat, you're dumped." Then that would be abgn*
If he gives you some time to do something about it, "You're getting fat, if you don't do something about it, then I'll dump you." Then that, at least, would be fair. It would then be your choice whether you wanted to do something about that. Do you want to be with him? Someone who ranks your size over all of your other qualities?
*bang out of order.