Well the number of participants, while related to "messiness" of combat, isn't its only factor.
For example, if I have a character Teleported or Feather Falled by an enemy, things just got a lot more messy.
On the other hand, if you get 8 enemies, but you also give me 10 allies, and you place them next to each other at the beginning of the combat and you set their AI to attack each other, that's actually not very messy at all. That's very ordered, because what's going to happen isn't only predictable, it's also very safe for the player's characters. It just takes a long time to process.
Messiness has more to do with the amount of possible variations a fight can have. It's reloading after losing a fight and having a completely different set of things happen due to a relatively small difference in the opening moves. The War of the Stones isn't what I mean by messiness at all; that one goes like clockwork to the point you barely even have to be there yourself.
