This is completely untrue. An average human is way too big to penetrate a vixen!
You'd think a chicken would be able to be penetrated and fucked like any other creature because of how big eggs they lay, but you cannot safely have sex with them either. I think there's a section here somewhere in the "how-tos and educational" why exactly.
Lionesses and tigresses are too small to have sex with, too! While it's a big animal, it's vulva and vagina is quite small.
There's a good reason why dildos for human females are not the size of a baby they give birth to!! Going out is an entirely different thing to getting in! And newly born fox kits are NOT THE SIZE OF A GERMAN SHEPHERD PUPS! They are more comparable to the size of a newborn kitten!
Oral sex is a quite safe option, I'm 100% sure you could satisfy a vixen with your tongue or a male fox with your mouth, though. Penetrative sex is not everything.
Oof, I have to join the "I'd fuck a huge bunny" club o/
I guess a huge fox would be cool, too.
Rodents could be interesting, also. But tbh, rodents are pretty scary when they're big, a giant hamster would probably just fuck everyone up and eat them