I agree that this is the key dynamic making this happen. Except it’s worse than that. Imagine that you enter the room or exit a conversation. Out of the options listed above, you are indeed probably going to pick #2. But which existing conversation will you join? Unless you’re going out of your way to keep group size small you will likely be influenced by two things: (a) prefer a group containing someone you know/like; (b) prefer a group that looks like it will be glad to accept a new person. Both of these favour larger groups over smaller. So larger groups will grow faster.
I agree that this is the key dynamic making this happen. Except it’s worse than that. Imagine that you enter the room or exit a conversation. Out of the options listed above, you are indeed probably going to pick #2. But which existing conversation will you join? Unless you’re going out of your way to keep group size small you will likely be influenced by two things: (a) prefer a group containing someone you know/like; (b) prefer a group that looks like it will be glad to accept a new person. Both of these favour larger groups over smaller. So larger groups will grow faster.