IIN to want your gf to lose weight (fitness) so she is better in bed

You know, my first instinct was to be all, "Rawr, you don't get to tell your partner how much she can weigh! How dare youuuu!" But after I read your post and not just the title of it, I think you do have a point in wanting her to become healthier. You don't seem like you're fat-shaming or body-shaming; you seem concerned about her ability to lead a normal life, and I think that's entirely normal.

Her getting winded from sex might not be about her weight alone, so the first thing you and she might want to talk about is scheduling a doctor's visit for her. It's possible to be overweight and healthy, but someone who can't catch her breath when she's on top isn't healthy; she needs a doctor to find out if something else is causing that.

Maybe you can try some activities that build stamina and strength outside of sex. Is she up for going for walks in the park or strapping on a pair of skates? Do you both own bikes? Those could help her physical fitness level in and out of bed.

Thisissomuchfun is right; you need to approach any discussion of her weight very carefully. I don't know that there's a way to say "Honey, you're too heavy to enjoy sex fully, and it worries me" and not make her cry, but framing it as something you can work on together can help.