Cats can really range in weight. I have a 8-9 month old DMH and she is 5.5 lbs right now and I doubt she is full grown or full weight yet. She looks fine and eats well, though she was malnourished as a kitten. I took her to the vet last week and asked if she was too thin (I can feel her pevlic bones) and they said she was fine, just a small, thin cat. My 2 year old DSH is 9 lbs and that is how the vet wants her. She has a smaller frame and should not weigh more than 10. My 3 year old DSH is more like 12, but as a boy he's larger and lankier than the other two. He's also got a thicker head.
As long as the cat looks normal, it should be fine. If you're not sure, take it to a vet. Being overweight is very bad and can cause serious health problems. Nothing gets me more bothered than fat pets, since it's so easily preventable and the animals depend on us humans for their health and nutrition.
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