Why does it take less time for small rocks to wear away than it does for large rocks to wear way?

If you take one large rock it only wears away on its surface.

If you now break that rock in half the surface is the old outside AND the new break in half - therefore more surface area to wear away. Break these two halves again and there is more surface area to wear away.

Keep going and break your large rock into many pieces and you get small rocks which wear away much quicker.