The former dwarf the latter have kerb stones and are not composed of stones to anything like the same extent . It is this differece that could explain why some cairns were removed i.e. a useful source of stone .Furthermore none have been shown to cover the capstone suggesting the possibility that not only stabilising the orthostats the purpose may have also been to help the raise the capstone .
Agree with the source of stone point.... clearly Iron Age culture generally respected/feared cairns as representing a bygone source of 'power' or else so many round cairns and long/round barrows would not have been retained, and still survive, within hillforts. However once that reverence had gone, great source of building stone! And the nearer a site was to later habitation, the more chance of complete removal.
The thing that bugs me, however, is the hollowing out of upland cairns where it would have been just as easy - if not easier - to collect surface rock straight from the ground lower down the hill/mountain. A good example of this is at the Carnau'r Garreg Las I visited a couple of weeks back. Rock litters the mountainside, yet both cairns are substantially defaced internally. The 'storm shelter' theory is also tenuous here since the cairns lie well off the main east/west path traversing Y Mynydd Du, with some (albeit isolated) farms less than an easy hours descent to the NW? Such mutilation would have required a great deal of effort.... for what? Treasure hunting/unrecorded excavation would appear the most logical explanation.... people will be willing to go to lengths if there is the chance of substantial reward. Or could some ritualistic requirement have provided the necessary motive for destruction... sort of akin to incomers to an area saying 'your justification for ownership is your ancestors reside upon the hill.... well, they don't any more since we've got rid of them... we're in charge now'.
