http://fixyt.com/watch?v=bPSElw8qEsI
http://fixyt.com/watch?v=lv5BtJMKamM
Getting competitive: http://fixyt.com/watch?v=tvT5LMhvogw
And a non-Alaskan entry:
http://fixyt.com/watch?v=0zDo7hkmCNY
If you're landing on a wide range of terrain, much of it unimproved, soft, and/or uneven, large tires will float you over the soggy stuff and smooth out the lumps (large wheels experience less shock with terrain / surface variation than small ones)