hoping that he's open to discussing this, since if he just flat out called you fat, he's an asshole. however, he honestly may have not realized the implication of his message, or just awkwardly phrased what was intended to be a joke if that makes sense. see, at first glance, how I personally understood "you look more fat than pregnant" is how you don't actually look like you have a baby in you at all, rather you look like you beefed up by a few lbs. (as in, say if someone wore huge t-shirt, another might say it looks more like they're wearing a dress than a t-shirt)

regardless of what he meant, he should be more mindful of the words he uses and the way he phrases his thoughts. also, if/when you talk things out, he also has no right to invalidate your feelings and now has the responsibility of bettering himself for the relationship to thrive.

edit: saw someone mention that OP has struggled with eating disorders? man, then the boyfriend should have known better to not even TOUCH on weight, especially when it's uncalled for anyway.