Don't shoot the messenger - just passing along one author's opinion. Your question was how to tell the difference between large and small rotor units - presumably without disassembling them. The 6V has angled mount holes - very obviously different. The fact that the large rotor units move more air per revolution as do the 8-71 and up is a given. They're going to be more efficient on large displacement engines.
What, exactly, are you concerned about?
