Lots of couples are fully committed to aach other without being wed. But Marriage really does matter to some ppl. Lots of ppl. And when there's an imbalance between what one partner wants and the other then its a problem. For many people getting married clearly shows their commitment to each other. And not marrying is a sign that the other person doesn't want to commit. Which, in a way, is true.
Here, there's not much to go on tbh. But it looks like OP wants to be married to her DP. He does not want to be married but she's not sure why.
Maybe he is a serial monogamist, always waiting for the next, better thing to come along, always leaving his options open but enjoying the comfort that a "LT" relationship brings. Maybe he just doesn't think marriage is necessary and wants, still, to spend forever with OP?
Only he knows, but it doesn't seem as, though the OP knows.
Being overweight, it would seem, is usually a result of emotional overeating. Knowing about diet and exercise is not enough to curb our emotional drives.
Being made to feel bad about your overeating does not help, it can only make things worse and is unkind.
Maybe he is just rubbish at making jokes? There's not enough information about the context of the conversation to know whether he was playfully teasing her and loves her chubby bits, but just isnt the type to formalise his commitment, or whether what he was saying is: "I don't want to promise to stay with you forever (because you are too fat/ because I like the freedom of not being married) and i would like to keep my options open"
Fwiw, i am not overweight, but like many women i aspire to be thinner than i sometimes am. This only ever happens when i am relaxed and happy enough to never need to comfort eat.