I know what you're thinking, but she's been this way since a few months after I got her (a year ago) and she's been to three different vets several times (all she ever had was an infection or two). She did have an ear infection once; the vet said that that can make them dizzy when they walk, so they're less active, but she's been rid of it for a couple months. Blood sugar always over 90, never had a blockage, nothing swollen in her abdomen. She's also extremely lazy! But she doesn't seem out of it like my insulinoma ferret was before treatment. I don't know what to do, she's definitely too fat and seems uncomfortable when she squats to poop (and sometimes get dirty...gross). She lives with her brother so it would be difficult to limit her food intake without messing him up. He's extremely active. I've been trying to engage her more to get her more active but she gets bored easily. One of my other ferrets sometimes tries to get her to play by jumping on her, but then just feels her belly like he's fascinated by it. It's funny to watch.

Btw she's about 5 years old. I remember thinking she and her brother were skinny when I adopted them from the shelter, but he's put on just a bit of weight and she's ballooned up. Don't remember how much she weighed but I think it was under 2 pounds, and she was 2.3 last time I checked. I guess I'm mostly concerned because she has a "pot belly" body shape now, whereas another one of mine with similar weight gain has it distributed more and is generally softer. 

tl;dr Ferret is too fat but otherwise healthy.