Ferrets come in all different sizes and body shapes.  A healthy adult
male is normally anywhere from 2 to more than 5 pounds (900 g to 2.25
kg), and a female from 0.75 to 2.5 lb (340 g to 1.1 kg).  Ferrets,
especially males, normally gain up to 40% of their weight in the
winter and lose it again in the spring.  Some ferrets are naturally
"chunkier" than others, too.  When you run your hand down your
ferret's flank, you should feel his muscles ripple a bit and be able
to feel the ribs, but they shouldn't stick out or feel too bony.
Small "love handles" are common in the winter.  If he feels soft and
"mushy" or looks pear-shaped, he might be overweight, or just have
poor muscle tone from insufficient exercise.

If you think your ferret might be overweight, make sure he doesn't
have some other health condition that makes him appear overweight.  If
the weight isn't evenly distributed, especially if you feel a large
mass or a number of smaller masses in his abdomen, he may have an
enlarged spleen.  He might also have heart disease which is causing
him to retain fluid in his abdomen [1.1].  Unless you are absolutely
certain that he is simply overweight and does not have another
condition, please take a trip to the vet just to be sure.

If your ferret is indeed overweight, he needs to eat a "leaner" food
and get more exercise.  To reduce his calorie intake, mix his regular
food with a high quality food for cats (as opposed to kittens) or
Totally Ferret for Older Ferrets.  You still want to keep the protein
and fat content relatively high, but not quite at the top of the
recommended range [6.1].  Mix the new food in gradually so he accepts
it better.  Of course, also reduce the number of high-calorie treats,
especially sugary ones and those designed for weight gain (NutriCal,
FerretVite, etc.).  To give him more exercise, make sure he's not
spending too much time in his cage, especially since many ferrets will
eat when they're bored.  Play with him as much as possible,
particularly games like chase; if he enjoys going outside, consider
taking him on a short walk each day.

If your ferret is underweight, there's probably some underlying
medical condition.  In addition to the obvious diarrhea and vomiting,
many diseases can cause loss of muscle mass, especially in the hind
end.  If your ferret seems to be eating and he's still underweight,
take him to a vet to find out what's wrong.

On the other hand, perhaps he hasn't been eating because he's been
nauseated, congested from a cold or allergies, or stressed from some
change in the environment.  He might not like a new food, or the bag
he's been eating from might have spoiled.  If he isn't eating and
you've recently changed something, try changing it back; if that
doesn't work, get him to a vet right away.

"Duck Soup" [12.8] and other things [12.7] have been suggested as
good ways to put weight back on a recovering ferret or to help
persuade a ferret to eat.