Yeah, it's very likely that he meant very little by it, but my own experience and that of some women I know (and looking at many threads on here) THAT is the more likely scenario. He is negging her. Putting her in her place. Making sure she doesn't get ideas above her station. 
I was wolf whistled at in my 20s, by 2 men who shouted 'you're gorgeous blondie!' when I had a 'figure hugging' dress on ... I was out for a meal with a boyfriend (of 4 months at the time.) I had just gone to the toilet while we were waiting for our main course, and was on my way back... And he was like ...
I just rolled my eyes at the men and went and sat down with him... About 10 minutes into the meal, he suddenly decided to tell me that the dress was nice, but might look better on a slimmer figure. I was like 
I said 'what?' He said, 'oh don't worry, you look fine in most of your clothes, but that makes you look a bit fat.' I was 9 stone 3 at 5 ft 4. I was not 'fat.' I said 'no it fucking doesn't.' He said 'well if you want to tell yourself that. I'll leave it with you.' Then he carried on eating his food and started talking about something else.
I sat in silence throughout the meal, as he tried to talk to me. I was giving one word answers, and barely making eye contact.
Even my own MOTHER told me I looked 'cracking' in the dress, and that any man should be proud to be seen with me, and my dad said I looked like Marylin Monroe. The compliment from my mother was rare, so I must have looked OK!
tl;dr, I finished with this bloke shortly after. It wasn't the first time a date or a boyfriend had critisized my weight - and even called me chubs, lardass, tubbytum, and similar 'petnames!' When I was less than 9 and a half stone.
Honestly, no wonder so many women have eating disorders.
@CurlingChamp YANBU, Your DP shouldn't have said what he said. I would tell him I was pissed off. Maybe not a dealbreaker, but would be if he says anything like that again.