Honestly? If I were ever in a relationship and my SO told me to lose weight because it would give them a more aesthetically pleasing body to fuck, I would tell them to go fuck themselves. But that's just me. Maybe she's going through something that's caused the weight gain. Don't be a dick about it. The end goal should be about her getting healthier (if she's even unhealthy. Just because she's got some extra pounds does not by any means mean she's unhealthy). Just have patience with her. She is under no obligation to lose weight so as to please you. If it becomes a problem, then you reevaluate your relationship. Don't make her change herself to please you.