Horse Power plays it's part. HP is importaint for sustaining momentum/work under a load but the torque is what gets it there and the more you have the more prop you can swing. Then 1+ HP can easily keep a 13 even a 14 inch prop spinning near peek RPM for a 4s, while the equivilent 2s might turn that same prop but it will never reach peek RPM due to insuficient torque. Thus we reduce the prop size for the 2s and rely on the higher RPM to produce the same thrust with a smaller prop. Esentialy when you chose your engine, 4s or 2s, you are trading HP for TQ or vise versa.