Wow, why do men think it's OK to comment on a woman's weight?
When I was 17-18 ish, I was 10 stone (5 ft 6,) and a size 12, and this bloke I was seeing kept calling me 'chubster' and 'little lump' and saying my arse fills the couch' and he 'loves my little pot belly' and he laughed as he was saying it. Coz it was soooooooo funny. 
Seriously, I was fucking slim! I had a 26" waist.
After a few months, he started to comment on what a great figure a female colleague of his has - she was 7 stone and about 5 ft 8. It was clear what kind of woman he wanted - one who was very thin.
I am not having a go at her, and would never attack anyone's body shape, and she was not the problem anyway! And I had had enough then, I had got sick of the comments about my body, and I didn't even like him anymore. So I said 'just fuck off and date her then if you prefer her' and stormed off.
He called me 3 or 4 times, but I ignored his calls. Then he stopped trying, and I never saw him again. No loss.
Your DP may not have meant it as 'offensive' but any man who takes a pop at his woman's body shape and size is a rude twat who is trying to make you feel insecure, and deserves dumping if they keep saying it. 'I like your plump thighs' is not a compliment. It's letting you know you are carrying extra weight, and he obviously doesn't like it or he would not comment. (IMO.)
I don't know why they say things. Maybe to keep you a little insecure and grateful that he is attracted to you. 
Whatever the reason, it's out of order, and not acceptable.