I was wondering if someone could explain how exactly guild leveling works. In Cataclysm, I will be in a hardcore 10 man raiding guild but it made me think that with a guild of only about 12-15 members, leveling the guild would be extremely slow... Because, it would make sense if you gain a lot more guild xp with more members, but at the same time that would be unfair to smaller guilds. There are so many different factors to consider so I'm wondering how exactly guild leveling works and what it is based on.
There's a daily maximum of Guild Experience you can earn as a guild. This is fairly high, but it all depends on how active those 12-15 members in your guild are. The maximum experience was also implemented so that guild with 300 members didn't have an enormous edge over the one with 25 members and could just get level 25 straight off the bat.
You get guild experience from doing dungeons, quests, battlegrounds, raids and so forth. I wouldn't worry too much about it if I was you
Guilds with more members will get more guild exp, Blizzard seems to want people to move towards bigger guilds. Not saying that it is impossible to level as fast with a small guild (Im unsure), but bigger guilds will generally level faster.
bigger guild will level faster, this is a fact. Smaller guilds will be able to get to level 25 but not as fast. There is an XP cap per (unit of standard time) that will prevent XP zerging the first day but that will not change a whole lot in the long run. Most active guilds of 15+ members should progress rather quickly.
Keep in mind that you need guild rep to get most of the rewards for your character anyways (rewards, not perks). Even if the larger guilds level up faster then you do, you could still get guild mount or new heirlooms sooner if you grind the rep needed faster
Guild leveling is based on Guild achievments, those give allot of guild XP. So if you got a strong group of players and get those achievments very fast, You will level faster then those big guilds etc. Its harder but do-able.