chip said it on another post - it's mostly to do with stroke length.

the longer the stroke, the further the piston has to travel for a given engine speed. obviously at the ends of each stroke, the piston has to slow down and then accelerate in the opposite direction again, and the further the piston has to travel in each stroke, the faster it has to go to cover that distance in the same time so the acceleration has to be much greater.

it's the acceleration that puts components under high loads and stresses, rather than the speed.