well maybe he was a wild caught gecko and has parasitews. parasites will eat all his nutrition befor he has a chance to digest it and store it in his tail. it is very good that he eats but you may want to take him to a vet to get a stool sample. he probably did loose his/her tail. this is a picture of duder (my first gecko) he is obviously not skinny but he has lost his tail and that is why it is bulbous.