honey, I'm going to say this from the perspective of someone who is your mother-in-law's size.....
No one should judge or measure (no pun intended) anyone else, their abilities or their character, based on size. Therefore, she should not be saying anything negative about your size or even making comments that you need to lose or need to gain, etc. You are you, and she should love you for who you are.
Now, I will say this: you make a great point of saying what sizes she wears and telling us that you are a "normal" person. Hmmm. What does that make me? Abnormal? Ugly? Lazy? There are a few unspoken comments in your question about fat people, too, and they aren't nice.
You clearly feel superior to her because you are smaller physically....when you should be more concerned about how to deal with your mother in law lovingly and being the "bigger" person emotionally.
So I suggest that the next time she says something you sit next to her (with no one else around) and take her hand and smile lovingly and say, "You know, it really hurts me when you say things about my weight. I want us to love each other for who we are, not what we weigh."
Saying "what WE weigh" means that neither of you is perfect and neither should judge the other.
If she doesn't take the hint from that, then you may just have to ignore it. She's going to be his mother forever, but you don't have to like her. Just smile, be polite, and be a bigger person.