I have 3 cats thry are all different heights, weights and chubby lookingness. Like with dogs none purebreds can look quite different to each other. I don’t go by weight with any of my animals since even with a purebred their can be quite a range of healthy weight. Can you feel his ribs or spine? You should be able to slightly with out much presser.
This is the sort of thing I use. But keep in mind that some cat have thicker coats then others, which can make them feel or look fatter slightly and some are built a lot chubbier looking then others.
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