You may be focusing on the wrong spec. What is important for a fan is NOT the RPM, but the max AIR FLOW in CFM. Not surprisingly, a much larger diameter fan can blow much more air at the same RPM. BUT to run a such a high RPM, a large fan will require more power, make more noise, and be harder to balance for smooth operation when it is being made. So, you will not find 200 mm fans sold with the same max speed as 120 mm fans.