there is no "correct" weight,all cats,as do humans,vary in weight and size. If you have cats and are concerned about their weight,pop them to the vets where they can advise you on a good weight for your cats.
Thanks Rio116 - it was conflicting views and comments from the same veterinary practice that caused me to raise the question. At 4.28 and 5.2 kg each though apparently this is nothing to worry about. Thanks again.