There was a local belief that the stones were gradually sinking and shrinking. When the Rev. Francis Kilvert visited the site in 1878 he noted in his diary:
Joseph Gwynne told me that when he was a boy the great stone called Arthur’s Stone was much longer than it is now. A hundred sheep could lie under the shadow of it. Also the stone stood much higher on its supporting pillars than it does at present, so high indeed that an ordinary sized man could walk under it.
Across the green lane and opposite the stone was a rock lying flat on the ground on which were imprinted the marks of a man’s knees and fingers. These marks were believed to have been made by King Arthur when he heaved the stone up on his back and set it on the pillars.”
No, I don’t think the last bit quite makes sense, but there you are. I suppose it does imply people thought it was originally part of the main tomb? Diary quoted in Jacqueline Simpson’s ‘Folklore of the Welsh Border’ (1976).