Remember that ratings are given under optimal conditions and normally with 15% or higher Nitro content.

That said, your .61 is doing OK for an 11x7 prop, and it appears the 12x6 is bogging it down... however the latter may be due to mistuning.
With the different prop you need to modify the tuning.

If you saw a net RPM increase WITHOUT re-turning the engine, then you likely had it too rich to begin with.