Kinda goes back to full scale. On full scale the main gear supports almost all of the weight of the plane. The nose or tail wheel is there to keep it from falling on it's nose or tail when parked or moving. Less loading requires a smaller tire. Look at the HUGE airliners. BIG multiple truck landing gear assemblies and one spindly little nose strut with maybe 2 relatively small wheels on it. There would only be one but they are big on redundancy in case one goes flat.