Don't worry about your size

there's a difference between happy and healthy. She isn't that bad, so I would say she doesn't need to worry much yet- but while it's good to have confidence, it's not good to end up too heavy to walk on your own.

I still don't know very many fit people above 250 lbs....

Muscle weights a lot. Think, like, bodybuilders and stuff.

Size is a good indicator of how much fat someone has, but the fit look varies with each sport and lifestyle. Also, there are plenty of skinny people out there who are very out of shape.