In reply to Stephen R Young:
I don't think it's anything to do with size - they are simply two different things. A cliff (crag, buttress etc) is formed through glacial or fluvial erosion, earthquake, landslip, weathering etc and is 'attached' to the land, while a boulder usually comes from bits breaking off a cliff (though sometimes transported a long way from the cliff as an erratic) and sits 'on' the land.