The Theravada forum seems better -- not just in quantity, but also in quality.
Shouldn't it be the other way around?
I would not say that the Theravada forum seems "better", it is different.
In what way is it different?
It seems to be more focused and the reason for that may be that there is (only) one common scriptural basis to refer to when discussing subjects. This sole scriptural basis makes it easier to focus, helps to reduce distraction and also entails that there are more experts as to this basis.
I feel that it is much more difficult to find a common basis in the Mahayana forum since Mahayana is so diverse.
Actually the total amount of user/practioners required for a Mahayana forum would be "Mahayana schools x (amount of users in a Thervada forum)"