I think the author is reading too much into this. Even in ancient times it was known that hybrids were frequently larger than the parents. Take Ligers for example (lion tiger hybrid).

People were probably more familiar with animal husbandry than today, so it makes sense to exaggerate that natural tendency to make somewhat believable monsters.

I don't think it had anything to do with xenophopia or "category violation" . The Earth has all kinds of weird shit like flying squirrels and humans