Coflein cites several enigmatic unclassified ‘mounds’ upon the summit plateau of Pen Trumau. In my estimation these are not marker cairns (there are several obvious examples of those upon the main track) and appear to serve no purpose other than forming a diminutive prehistoric cairnfield. This example is at SO2036529304.
Image credit: Robert Gladstone
I think that there are many, many cairns in the UK currently wrongly dismissed as 'just' marker cairns when their purpose is as you have stated here, and are millennia old. Will upload images of two from Ramsey Island and near Dunman, DnG when find my round tuit. Size should not be important.
I'd forgotten about these until I came across them on a memory stick and thought 'where was that again?' I completely agree. When you know an area well it becomes obvious where cairns don't fit the 'walker's' or 'marker' label. I'd wager (from experience) that 99% of walkers up on The Black Mountains - and elsewhere in Wales - stick rigidly to the main paths. Miss all the fun that way.
Yup. My threepennyworth....infant death must've been so common back then...a child of a family of status needn't necessarily have had a whopping great cairn on top, just something visible on the skyline, enough to cover remains or ashes. I really must put up the images I mentioned, haven't yet because haven't been in fieldnote mode - I went to Ramsey ten months ago ffs! - and there's sites I've been to that really deserve the full works not just pix. But the diddycairns when you see them are very, very old, not clearance related and certainly not in walker or tourist hotspots. Soon.