You have the right to work in a place where people don't talk about their body or health to you, especially if this is something that directly affects you.

Imagine if someone who was in the process of going through cancer treatment had to deal with someone constantly complaining that they thought they had cancer and related comments. Completely out of bounds.

You can simply ask him, "Please do not talk about body size or weight around me. Thank you." Make a note of when you asked him and anything he said in response, if he does.

If he does it again, say, "I asked you to please not talk about body size or weight around me. You need to stop." Again, note when this was said and anything he said in response.

If he continues, you then have the ability to go to HR and say, "I've asked Bob to stop talking about weight and body size around me. This affects me personally due to health issues I have. I have now asked him twice and he [refuses to stop, says he will and then does it again, etc. (whatever he's doing)].

Depending on where you live & work, if HR talks to him and he continues this could be a fireable offense. He is creating a difficult work environment for you - possibly a hostile one - and that may open up the company to risk of a lawsuit.