I remember, oh yes, I remember all the speculation before the expansion hit about the new guild rewards system and what that would mean for all size guilds. I think that after all the speculation, and now about a month into Cata, I've come to realize something...size matters, sort of!
To test my theories I have taken various toons of mine and found guilds of different size and purpose. Regretfully, however, I haven't played my DK much, and so an invitation that was extended to me here on my blog to join a certain Llane realm guild has not come to fruition. But if I ever do log into my DK I'll do what I can to contact that person (you know who you are).
I have a couple of my toons at level 85 and are in a raiding guild. There's approximately 70-80 characters, with about 50+ accounts, and currently we stand at the precipice of guild level 8 (as of today we're at 7). I dropped one of my son's toons, a level 82 DK, into a guild with more than 740 people, and at the time they were close to level 9. I haven't checked their level in the last few days, but they were making very good progress.
I have a lvl 25 mage (and leveling fast) that I threw into a guild with 5 people. Right now we're still at level 1. Ugh. And just the other day Gronthe joined a casual, leveling guild with nearly 200 people that currently stands at guild level 3. Of all these guilds, only the 5 man guild is inactive. All of the others are active in their own way. My raiding guild has, on average, 4-8 people on at the same time most of the day and into the night. My son's guild (740+) has about 40-50 active on at any time of day. Gronthe's new guild is also very active, about 15-20 on most of the time.
Size, it seems, has an impact on guild leveling, but only to the point that those people who log in actually do things. My raiding guild, albeit somewhat smaller and not everyone has even played since Dec. 7, seems to level fast despite smaller numbers because everyone is extremely active and doing things to progress their character, in terms of leveling, professions, pvp, dungeons, etc. Nobody just sits around and chats in Stormwind like I've seen in the other guilds. To be fair, the other guilds that I have mentioned are "casual", i.e. not particularly interested in raiding or maxing out professions or things like that. I'm just making anectodal observations, that's all.
I happen to enjoy the different guild I'm in, and all for different reasons. They each offer me what I seek for those particular characters. Serious on one hand, nothing but ridiculous fun on others. I think the only exception is the 5-man guild I'm in with my mage, I think I'll look for a bigger one. I enjoy the perks, it's nice to not have to pay as much to repair gear, really nice.
I don't have the resources to gather the appropriate information to make a proper analysis about which guilds are leveling the fastest and what is making them do so, but if I were to guess the answer is simple, ACTIVITY. Not just logging in but DOING things. Some guilds don't need all the guild perks, so to level the guild slowly because you enjoy chatting or telling jokes or going off to do silly things in your own time is perfectly acceptable. Guild perks are nice, but only if that's what you seek out of the game. If you enjoy leveling slower (which is difficult nowadays), you don't want the bonuses you get from guild leveling, so you may just wish to join my 5-man guild. :)
Take anecdotes for what they are worth, just one man's story from his own perspective. I still have some guild joining to do, that's for sure, I'd like to spread my toons out when I can, just to get some flavor in my game experience. So, although larger guilds have an advantage, they can only capitalize on that if they actually do stuff. But, like I said, not all guilds are the same. A smaller raiding guild with only 4-8 active people at a time can level more than twice as fast than a guild with more than 2-3 times the about of logged in players if they want to.
I hope that everyone is enjoying themselves, doing what they want, and in the guild that brings them the greatest utility (and/or enjoyment). If you're not in a guild, and not by choice, I wish you luck in finding a home that suits your style. Be active, but more important than that, have fun!!!