Nine Hands said:But does it really matter? Unless you are going to run an encounter with creatures larger than the devil, then the problem is moot.
If you are...well you are the GM, just do what makes sense.
I understand your frustration but I don't want to see a bunch of text explaining how the effect won't work on a creature that is 2 size categories or larger. Its a waste of text since this monster is for fighting medium sized people.
IIRC, it's actually an Epic tier monster, so assuming that the PCs (and their allies) are all going to be Medium sized is a bit of a stretch.
And if I had a dime for every time I either played in a game or read about a game that had some kind of three-way conflict with monsters fighting other monsters I'd... well, I'd probably be able to pay for lunch, anyway.
Finally, it's not just this one demon that's the issue here - it's the fact that the whole game has been designed using the same philosophy, and I constantly find myself looking at stuff that makes me go "this doesn't remotely make sense". The only time I'm ok with being asked to turn my brain off to this degree is when I watch action movies.