Gerald of Wales passed the mountain in 1187*. He mentions the legend, and another:
We proceeded on our journey from Cilgerran towards Pont-Stephen [Lampeter], leaving Cruc Mawr, i.e. the great hill, near Aberteivi [Cardigan], on our left hand. On this spot Gruffydd, son of Rhys ap Tewdwr, soon after the death of king Henry I., by a furious onset gained a signal victory against the English army, which, by the murder of the illustrious Richard de Clare, near Abergevenny (before related), had lost its leader and chief. A tumulus is to be seen on the summit of the aforesaid hill, and the inhabitants affirm that it will adapt itself to persons of all stature and that if any armour is left there entire in the evening, it will be found, according to vulgar tradition, broken to pieces in the morning.
The myth of tumuli changing size appears to have been a common one: it features in Wonder 13. However, it doesn't seem particularly common now, perhaps because it is easily tested and tumuli tend to be quite large - indeed, it is hard to imagine anyone visiting such a site would find it likely. The legend may take its basis from the similarity, or even etymological link, between Tumulus and the Latin tumere: "to swell"1. However, it is also possible that what is being described is an open cist (Info) / kistvaen (Info). Although cists tend to be small, some can take a straight body: undoubtably the best known cist burial in Ceredigion is Bedd Taliesin (Info) which had a cist of ~2m (now largely collapsed)1. A cist would solve the obvious problem that a tumulus is too large, while still allowing a tumulus to be the location.