I don't have anything against the large groups, I just tend to avoid them because they're often very long running and have some intricate plot that only makes sense to the senior members. Take for example, The Great Forest. I don't have anything against them, in fact I enjoyed being a loner on their map. I just didn't enjoy being a queen in ThunderClan, as I felt like I just jumped right into the middle of a book, and I didn't quite get what was going on.
Now, I'm not trying to put down TGF, nor those who have ever been apart of the group. What I'm trying to say is that the plot seems overly controlled by the admins, and just too much happens. If you haven't been there from the beginning, it might not make sense.
As I said earlier, I have absolutely nothing against the larger groups. They're just a little too big with too many people, most of which you may never interact with, and often have some sort of plot that I just can't get myself interested in. Some are very player-run, letting you make your own plot, and I love that. But I want a group where I can become friends with everyone, and with larger groups, that's impossible.