First of all, reamers only cut on the outer tip, not on the sides!
If a reamer is cutting a few tenths big, run the spindle in reverse and gently hold a fine white Arkansas stone or Norbide stick against the flutes. This will wear the OD slightly and make the reamer cut smaller! This works on either HSS, Cobalt, or Carbide reamers.
If a HSS or Cobalt reamer is cutting too small, take a new sharp carbide lathe tool bit, and carefully scrape the inside of the flutes, pushing the burr outward and increasing the diameter a few tenths! This technique only works once on a new reamer!
There are many opinions about controlling reamers by increasing & decreasing RPM and feed rates, as well as the amount of material being removed, last but not least the material your cutting. Experience is the best teacher!
WidgitMaster