Its not that hard to understand, two points:
1) According to my sales friend at BMW, the 3 series is the most "bought" car of the BMW fleet, and the 5 series is the most leased. So the lease guys don't take the ownership thing as seriously as "owners" do. I know some might disagree, but I think in the majority of cases, this holds true.
2) 57% of all BMW's sold are 3's, so logic would say that the 3 forum will be the busiest.
I have been on forums for almost ten years now and moderate in one on model railroading. My experience is that they get divided off to quickly and then traffic is sparce amgonst them, better to have fewer forums that are "busier."