Rpm is just what the spindle turns per minute.what you need is the surface feet per minute for a better understanding on what to use for polishing. For example: if you run at 250Rpm on a 8” Lap and you polish at the 7” dia the surface feet would be 458 sf/m.
If you polish at the 4” dia, the sf/m is 262. So you can see there is a large difference of sf/m but you are at a constant 250Rpm. Does this make sense?