Small rocks weather more quickly than large rocks because, compared to their volume, the surface area of small rocks is

Small rocks have a bigger surface area than the big rocks, relatively speaking.

Imagine you have 40 boxes: if you arrange them together only some of them will be exposed to  the outside area, and of those most only on one side.
In comparison, one lonely box would be exposed on 5 sides : every side expect on the floor.

It's lithogenous i believe of course i looked it up.