Finally got to read some unread motorcycle magazines over the weekend, and found a very interesting editorial in the July issue of Motorcycle Consumer News (a bare-bones no advertising fairly technical motorcycle publication I really enjoy. The editorials tend to be very engineering oriented, and thought-provoking).
The topic was on gyroscopic effects of motorcycle wheels and tires. The biggest contributor to gyroscopic effects, mathematically, is diameter.
Could the benefit of a 21" wheel in the dirt be the increased stability provided by the gyroscopic affect of the larger wheel/tire at the slower speeds usually ridden off pavement, especially in conditions that tend to cause slippage or side-loading due to the rough or loose terrain?