The RPM max of steppers is determined by
1. Torque RPM curve. You can't go faster than you have torque to do so. That is the only place bigger motors would help.
2. RPM increases with DC voltage. A lot of small controllers use low voltage drivers (24 to 30 vdc). That limites the motor RPM to about 60% of it's capability.
3. Frictional forces in the machine will cause smaller motors to stall sooner (see #1 above)
4. Steppers have an upper RPM limit even if everything is perfect (voltage, size, pulse stream, etc) while it varies motors run at the recommended 10 to 20 times their rated label voltage can reach about 800 RPM.
5. Fine leadscrews on the smaller mills mean you have to spin things a lot faster. At 20 TPI even 800 RPM becomes 40IPM max.
6. Only way to gain speed if you have good voltage and adequate motors, is to change the leadscrew to one with a courser pitch or use belt increase to spin them faster.
so most of it is just plain physics and limitations of steppers, controllers and power sources.
Totally Modular CNC Electronics