FWIW I had the standard merc 10" x 14 on and had to keep the throttle back so as not to over -rev & destroy the engine. (I was getting 6.2K instead of the 5.8K It's meant to top out at)
I went to a Yam 11.25 x 14 (yes, it's a Yam prop & I have had to modify the locking arrangement slightly, but it does fit!) - by keeping pitch the same & upping the diameter by the bet part of an inch I lost about 1000 rpm by going up a massive amout of dameter. On the plus side it now doesn't "let go" at the slightest provocation....
I guess you lucked out and got a tad more grip to offset the extra drag. Just shows how prop selection can't be done 100% by theory!
Now, I am still c. 200RPM short of my top RPM. Be you interested in punting the 13" prop? PM me if so.