Worms can cause weight loss. Usually you see shrunken combs with molting from what I have read.
You may or may not spot worms in the poo. When you see them in the poo, it means they are probably infested badly. Some vets will run a fecal test for worms for a small fee, or you can just go ahead and worm whilst tossing the eggs as recommended.
There are diseases that cause shrunken combs as well...however, worms are so common in chickens that if you haven't wormed them it is a good place to start. Dehydration can cause the comb to shrink from what I have read. If the worms are getting most of the nutrition...maybe that would explain it, maybe not. Sorry I have never seen the shrunken combs myself.