Just think of the crank, connecting rod, and piston as an over-center linkage, like a clamp used in an industrial jig. As it moves across top dead center, the crank pin can move an inch, while only moving the piston a few thousandths of an inch. Of course all but a few thousandths of that crankpin movement is horizontal, while the piston can only move vertically, so the piston only moves by the "vertical component" of the crankpin's movement, and ignores the horizontal component. That is, the crankshaft has a "mechanical advantage", or force multiplication, of around 100 to 1 as it moves across top dead center.
However when the crankpin is at exactly half-stroke, the crankpin and the piston move equal amounts. The crankshaft then has a mechanical advantage of only 1 to 1.
