The larger the bore on the MC, the more fluid it will move for a given length of stroke. It will also take more effort to move the piston. A typical power brake MC might be 1 1/8" bore.

On the flip side, a manual brake MC might be 7/8" bore. It will move less fluid for a given length of stroke, but will take less effort to move the piston. So, you have to move the piston farther down in the bore to move the same amount of fluid.

Bottom line:
Big bore, short travel, high leg effort.
Small bore, long travel, lower leg effort.