yeah, if I have a roly poly cat that's still physically active, running around, climbing on stuff, etc., I'd want to keep an eye out to make sure it didn't get too much fatter, but I wouldn't go out of my way to make sure it loses weight – that just seems like it'd be a pain in the ass for both me and the cat. If it's fat but still active then I don't think there's anything to worry about, unless it was the result of a sudden change in weight.