I find the atmosphere in a room with 2, 3 or 4 people far more oppressive that a room with a 100. Probably something to do with being able to ignore people when they are a multitude, but things getting too personal when it's a small group.

I always found it very easy to concentrate in big spaces as long as I feel nobody cares about what I am doing, and that seems easy in a big room. I also don't care much about small distractions or noises, I always hear people complaining about getting out of the zone when they have to chat a bit with a coworker. It takes me a big effort to get into the zone when I am starting to work, some kind of a starting period while I load all the relevant information in my head, but once I'm there I can get in and out and attend subtasks or have a conversation with somebody with no problem.