Some people suggest that a male cat grows larger if he is not castrated until the age of one year. Is that true? I'm hoping that maybe some studies have been undertaken on the subject to either prove or disprove it.
The only study I have seen on size of cats related to castration says the opposite. If a cat is castrated in early age their bones actually get a tiny bit larger as it delays the closure of the growth-cycle of their bones. (THE EFFECT OF PREPUBERAL AND POSTPUBERAL GONADECTOMY ON RADIAL PHYSEAL CLOSURE IN MALE AND FEMALE DOMESTIC CATS)
If neutered before puberty the jowls will not grow as large though.
Having kept and worked with cats for many years, (I am pro neutering) I have found in my experience that cats neutered before age of 1year do not grow quite as big as those neutered earlier. Not all un-neutered male cats spray, however if they do it is extremely unpleasant. Neutering your cat is essential to curb this habit, along with aggression issues and unwanted kittens (far too many out there) and other health issues.