There isn't a single ideal weight for pets. Greyhounds are slimmer than St Bernards, while Siamese cats tend to be thinner than large breeds like the Bengal.
To get around this normal variation, vets use the body scoring scale, which works equally well for dogs, cats and rabbits. It runs from 1 to 9, where 1 is emaciated, 9 is obese and 5 is the ideal. If your pet scores below 3 or above 7, take them to the vet for a professional check-up.