I suppose there wouldn't be any harm in a vet visit. Nobody ever regretted knowing for sure whether there was a problem or not. However, if it were me, I would leave it be for a while, unless you really think it's an emergency. It sounds to me like your girl is putting all her energy into growing big quickly and your boy is just putting on weight. If you think she's hungry, I'd say just feed her more. Maybe she's going to be a really big adult. I can't see it for myself so it's hard to judge whether there's a problem. She might just be a particularly active/voracious individual.