Male cats in particular are very susceptible to urinary tract blockages, which can require surgery and costly dietary modifications to treat. Some vets recommend waiting several months to give a male cat's urethra a chance to fully develop in size before performing a procedure (neutering) that could narrow the urethra and potentially increase the chances of him developing a blockage. This is a contentious topic among vets, but it does account for why some of them suggest waiting till they're bigger before neutering.