Size is one issue, kv rating is another- most outrunners are no where near fast enough unless you were building a true crawler. That's why the few guys that experimented with outrunners got rid of the transmission usually, and hooked the front/rear driveshafts straight to the motor. Not always the case though, as the 3rd link below shows...
Some reading material:
http://www.rc-monster.com/forum/showthread.php?t=8536
http://www.rc-monster.com/forum/showthread.php?t=25847
http://www.rc-monster.com/forum/showthread.php?t=9793
http://www.rc-monster.com/forum/showthread.php?t=9906
http://www.rc-monster.com/forum/showthread.php?t=9871