I agree about asking a vet. The weight issue could be health related, neglect related or they could be two different type of ferrets. I have two ferrets. Lola is a Real Canadian ferret and is huge and fat, no matter the amount of food she gets. Cha Cha is a Marshall's ferret and is thin and petite and pretty bony.

Get them on a mixture of high protein cat food of ferret food, but most ferret food sold at stores is pretty crummy. I feed high protein cat food and my ferrets look and feel amazing.