>>718921208 (OP)
I've got a male cat who is really thin and can jump to the top of the kitchen cabinets, and a female cat who's slightly chunky, and can barely be bothered to climb up their cat tree. I don't know what the difference is, I feed them both the same amounts at the same time every day.
If she's sick she's been sick for the two years that I've had her, and acts fine other than just being kind of lazy.