At that point, you're probably better of splitting into even smaller, more self-directed teams.
I've also worked with larger teams (4-12) that are not committee-like and are very self-organised and self-directed. So I don't think it's inevitable. I'm interested in teasing out the reasons some teams go one way and some another.
Bigger than 12 or so - then the committee-like stuff seems to become more unavoidable.
(Just read this rather interesting bit on how the process at TargetProcess http://www.targetprocess.com/articles/agile50months/ evolved. They have a mini-team per-epic - which sounds a bit like the best of both worlds to me)
If you mean that they've all got to mutually agree on some working practices that are going to enable them to work together effectively - then yes. If you mean have the same outlook or skill set - then it certainly doesn't seem to be true in my experience :-)
In fact, I'd say the opposite is true if anything. If everybody has the same outlook and skill set the group tends to more rapidly moves into a committee like non-self directive setup from what I've seen. I'm guessing that's because they don't have to come up with techniques for handling dissenting opinion as much.