It's an interesting theory and I think there's some validity to it, but I've seen fandoms (or at least, mailing lists/Usenet groups) of 500 members or fewer where all kinds of wacky and immature behaviour occurred on a regular basis, and in many cases it was because the posters knew each other that grudges went so deep and bitterness festered. Whereas my Laurie King mailing list has over 700 members now, people on the list are chatty but fairly independent, and we haven't had anything close to a serious dispute, let alone a flamewar, in literally years.

Maybe it depends on the type of fans who are drawn to certain fandoms. And maybe it also depends on the potential of the fandom in question for disputatiousness. Nobody in Russell fandom argues about shipping, for instance. There is only one ship and there's no question that it's canon. That cuts back on a lot of potential animosity right there...