I always thought that there was no fat cats, then I got a cat.
Frankie has been on steroids for his asthma and this makes him eat more. I picked him up last week to put him in his cat carrier (trip to the vet) and he was solid - all belly.
Maybe Colin has actually just got fluffier (like my Princess Merlin) and under all the hair lurks a slim and healthy cat.
It is sometimes a shock to realise that these animals with such huge personalities actually have rather small bodies - and big appetites.
Tie him to your whirly washing line and make him go round and round.
Good luck.
Susan, Frankie de Tom Cat and Princess Merlin