Mine never eats from tins either - must taste nicer from pouches.

I would take them to the vet. One of my cats who has sadly passed now (kidney problems) was eating loads like your describing. After blood tests she was diagnosed as having Hypothyroidism (sp?). She was always meowing for food to the point where it drove us mad. She wasnt overweight either. So i would get them tested. Then, if they do have that and they are given medication, ask for a prescription. I paid £28 for 100 tablets from vets, only to find them online for about £10!! I can give you the website i used. i had to send them the prescription, but they were fast.