I was talking about a similar thing with my mum yesterday, she had one of her female cats neutered 34 years ago, and the vet tried to persuade her to have a litter first, apparently it used to be cheaper to neuter them after they had a litter - it cost her an extra £5 cos she hadn't had a litter. She was also told that they would never grow and would stay like a kitten all their life cos they were 6-7 months old when neutered and not fully grown - apparently that cat didn't, but I told my mum it was likely just a one off, it hasnt been an issue in any of our kittens that have been done, and I have met adults that were neutered at 8 weeks and they dont look any different size wise.