My three are all from the same litter and but quite different weights (just under 4kg to over 5-and-a-bit). But they're quite different sizes and shapes too. Pip (the lightest) is smallish and lean, Twist is a solid cat (no fat but a hefty amount of muscle. If he lies on your feet in bed its hard to move!), Timmy was described by the vet as being a 'long cat' so although he was heavy, he wasn't overweight for his size. All three can put on an impressive turn of speed when required!
I think it really does depend on the build of your cat, and not necessarily be as simple as a weight between X & Y.