I also have this mental quirk where I get quieter in larger groups, but in smaller groups, I talk a lot. On the other hand, I've been asked to speak in front of hundreds of people with no warning whatsoever, and I do it without a problem. It might have something to do with whether I feel as if I need to or should speak.
Other quirks? I have to check my Blackberry. If I'm out out of the house, and I get out of a seat, I have to do something to straighten up my clothing (think Capt. Picard, for you Trekkies). I also look both ways when exiting a room into a corridor, or when I find myself at an intersection of hallways. That might not be so much of a quirk, though, since I don't run into people as a result.