Originally Posted by AdamHeard

If you look at and handle 4" and 6" wheels side by side with roughtop tread, the trend makes sense. As the diameter decreases, the cleat formed by the rows in the tread becomes more and more pronounced. The more pronounced it is, the more it digs into the carpet (I believe).

We haven't proven this theory, but it has been shown that smaller wheels will get more traction (for roughtop).