There is only so much fuel/ air mixture an engine can draw through the carb and into the cylinder. If the reducer is such a size that this maximum has not been reached then a larger reducer will allow more air into the carb and hence get closer to the max the engine can handle given it's capacity and port timing's. More power!
So if a 6mm reducer restricts this flow. Going to a 7mm will give you more fuel/air and more power. If 7mm give's the engine the maximum air and hence fuel/air mixture available to be drawn into the engine. Then going to an 8mm reducer will not do much. No extra power.
Again. This is given the same port timings and sizes in the engine as these could well then be the restriction the engine has against attaining it's max power output.

Don't forget. Extra air passing through your carb will need more fuel ! So the bigger reducer will cause a greater fuel consumption.

Hope this helps.

Regards
BM