geometry
It's really simple geometry. Look at a small block. Lifter,valve, rocker, pushrod, all in the same plane. No side load (front to back of motor). The design of the big block (canted valves) takes everything out of the same plane and puts a side load on the components. More lift, more spring pressure, more side load. Never seen a Big Chief head, but if it increases the angle of the pushrod, more sideload yet.