Why would boulder weather slowly while small rocks weather quickly?

Answer 1

1) The most obvious is that a boulder is significantly larger than a pebble or smaller boulder or rock. This would give the illusion of a smaller pebble weathering quiker than a boulder, when they are actually weathering at the same rate

2) The way they are being weather can be a factor. If you have a boulder being hit by only water or air while a pebble is all over the river bottom and sand than the pebble will weather quicker because there are more forces acting on it than the boulder.

Answer 2

Boulders have a larger surface area to volume ratio, which means they have more exposed surface area for weathering processes to occur. Small rocks have less surface area in comparison to their volume, so weathering processes can act more quickly on them. Additionally, boulders are typically made of harder, more resistant rock types, while small rocks may be made of softer, more easily weathered materials.