Firstly, I do think there are some people who use those kind of superlatives just to describe a person to someone else, and they don't really mean anything nasty by it. I know that I am sensitive about this becuz I have had so many deliberately rude things said to me about my weight, and sometimes by my closest family members. I have even described someone by saying that someone was about my size, not meaning to put them or myself down, just as if I would say someone was tiny or petite or slim or average in size: there's nothing derogatory meant in that ...
But in saying that, since this does bother you, that is what matters most here. I like the idea of asking your DH to ask his father to nix the weight comments; this would be the simplest and least confrontational solution.
One time, my DH (yes, my own DH) told a friend that he didn't recognize her brother anymore, and she asked why? He actually said " because he's so fat now". Without batting an eye, she replied ... "you better watch it pal, or the next time you say something like that, you're gonna get a punch in the mouth, and you'd deserve it too!" Her brother gave my DH a dirty look and walked away from him, refusing to even talk to him, then or now ...
Yes, this is one of my DH's flaws and even though I have told him to watch his mouth many, many times, he still slips up sometimes. He always says that he doesn't mean it in a nasty way, and I tell him that the person you are talking about doesn't see it that way, and then he apologizes.
As for a women, you could say to your FIL, "Dad, you'd better watch it or you might get a slap for comments like that someday," and then laugh a bit. That way, it may sound like you are giving him advice, like someone else suggested.
In the past, rude comments hurt me a lot, and I would cry over them; and I would be so shocked that I was speechless. Now I'm trying to use humor to deflect the comments away! Like BARGOO says, some people are ignorant & rude and always will be, but I think most people would listen if you said something to them, especially in a joking way.
I am still amazed at just how many thinner people (and some husky men) think that overweight people don't KNOW that they are overweight and they need to tell us. Like, do they think we don't look in the mirror each day? And then there's the ones that actually think we eat every minute of the day ... 24/7.
I actually had someone say to me -- "why don't you just stop eatin'?" I just replied "why don't you?" becuz he was fat too, but didn't think he was! I told him "I can't stop eating for the same reason you can't, becuz we'd both get sick and die!"
Sorry for going on so much -- I could go on and on; esp if I told you all the stupid and rude things I have had said to me in my life, but I won't.
Hope I didn't get too off topic and something here helps you a bit ...