Hello, I was wondering if fullness does anything aside from just increasing stomach size, and if there's a reason to keep the 999+/100% fullness on the parasites that I have
Fullness is only for representation, so it does nothing else. Regarding keeping the parasites I can’t answer because it depends if you want them or not :P.
Parasite accumulation increases chances of night visits by the worms and lust attacks during combat. They also prevent incubation from other eggs, so in the end it depends if you want those effects or not.