I might agree that the 66 is the very best "one gun fits all" size, but I have never found room to complane about my 62 either.
I killed a moose with my 62 cal Flinter several years ago. I used 140 gr of 3F and a .600" ball.
The moose dropped like a deer hit with a 270.
That almost never happens with moose (as I am sure you know)

I have killed moose with 375H&H, and one with a 348 Winchester. The 62 was WAY faster to put the moose on the ground. Even better then the 375, (and I love a 375.) There is something to be said for bullet diameter, --- and the ball still weighs about 320 grains.
VERY effective!