Well besides eating a lot, my grandmothers cat had Feline Leukemia and she lost her fur, lost weight and had horrible breath. If its not Leukemia, it could definitely be something internally, which the bad breath is hinting at.
Diabetes, thyroid, organ trouble, endocrine diseases, infection, communicable diseases, maybe a bad parasitic infestation so they aren't able to absorb nutrition and need to eat a lot to get just a tiny bit of nutrition which will also effect the organs over time. OTC dewormers are extremely dangerous often resulting in emergency and death and aren't effective on many parasites which require different treatments and if your kitty has something wrong with the liver or kidneys or something it would be a really bad thing to do so please don't try an OTC dewormer instead of seeing your vet. Your vet will be able to narrow the possibilities after an exam and history and talking to you about what is going on but your kitty needs a vet. Hope that baby is feeling better soon.