Could the kitten food make that much of a difference?

You don't say what food your mother has been giving her kitten? Yes, a good quality kitten food can make a big difference but Hills is by no means the only good food out there. I would say that 2kg is larger than I would expect an 11 week old kitten to be though, so be careful you aren't over-feeding! When you visit your vets for the kittens vaccinations, discuss with them and they will be happy to advise on corrrect amounts to feed and will let you know if your kitten is a bit too tubby!