I am currently bored, so I shall do some research!
There are 2003 accounts currently in a clan at this time. Out of those, 23% are in the top 5 clans and 36% are in the top 10. There are 95 different clans (including those at 0 membership), so there is an average of 21 members per clan. The biggest outlier in this data range is currently Trollitus Maximus, with 7.2 times as many members more than the average clan.
But what about clan "influence"? That is harder to measure, but I would like to try in a very basic measurement, by measuring how level level 50+ accounts each clan has. I'm not quite bored enough to search for every account that has a membership tag, otherwise I would add those in as well.
Out of the top 20, The Roleplayers' Realm and the Lost Wolves had the least number of level 50+ members, at exactly 3 and 2, respectively.
Going down the list, the first clan to not have any level 50 at all would be the Mongols. The second would be ᵀᴴᴱᗪᏒᎯᎶᎾNS.
With 531 level 50+ accounts in clans, total, 19% are in the top 5 (104 accounts) and 33% are in the top 10 (180).
Going over these numbers, I am actually a bit surprised. I had thought that they would be more lopsided in the favor of the "mega-clans" up in the top 10!
It's actually exactly 20. But other than that, I agree with you. My study was not designed to show any level of skill. Instead, it was designed to show a more abstract idea.... "influence." If anyone else would like to try their hand at quantifying that, be my guest, but until then, I stand by my data. ;)
It's interesting, and the best solution I could come up with in five minutes but it's not very good. The players with 'influence' are active on the forums, yet many forumers are sub 50, and many 50+ are not forumers.
Perhaps Influence can also be measured by 'Activity' Many of the big clans such as Maximums Trollitus and Lost Wolves have lots of Inactives, Obviously the more inactives a clan has the lower influence they have. My clan Vitrix Mortalis current have about 14 inactives but all are about to be kicked within the next few days if they do not reply to us, I only say its an additional thing to measure by because if we were not to annually kick our inactives we would easily be in the 200's of members by now.
So in a way this goes alongside Turtles point that levels mean nothing since a very active small clan is much better than a large inactive one.
Another thing you could measure influence by is rather than 50+ why not measure clan's members into bands of levels, Like 1-10, 11-20, 21-30, 31-40, 41-49, 50+. This would be a better way since many clans have lots of low-mid ranged players and few high leveled or just only high leveled.
Put these two data's together would probably give a greater detail into a clans influence, If I was bored I'd waste my time doing this but since I'm not just an idea to pass to whoevers reading.
Also this data from these two measuremants would show types of clans, the 'Invite all clans', 'Community clans', 'Elite Clans' and the 'Roleplayers'
true. Still holds my point regarding influence on the community. Most mapmakers are clanless, and most mapmakers in  did not publicize their maps yet though ;P i guess only Qi made a significant amount of quality maps, but most of those are not very often played on..