My 3 legged boy Monty was 5.4k and that's with one leg less
He went on a diet from the vets, not easy with a multicat household. He got weighed every month at the vets where I got 'told off' that he wasn't loosing fast enough.
Monty was withdrawn, and even with 3 legs became adept at getting food put out of his reach. In 8 month he lost the .4 bit and weighed 5 kilos.
2 years on with no diet he still weighs 5 kilos, a new vets now and they don't have an issue with it.
We did encourage playing, bought one of those runaround track things which brought him back out of his shell.
The original vet just looked on a graph and said he should weigh 4 kilo which when we rescued him after his appalling injuries he weighed 4.5 after being in the vets for weeks. He is a big cat, 4kilos would look awful on him and he would be soooo unhappy