So it seems like they're quite big for their ages then?
I thought as much, as wiki says an average adult domesticated male cat weighs 4.5kg, so it seems like I have lumps for kittens! I'm glad though, as all three were severely underweight when I got my paws on them, Ben and Jerry were put at 7 weeks by the vet when they were actually 10, and Archie was the runt of his litter and severely malnourished. I'd rather they be a bit on the dumpy side than what they were when they came to me!