Circularity, or an approximation of , was clearly important for a group of monuments i.e. causewayed enclosures , henges ,passage graves , stone circles ,ring cairns ,kerb cairns in contrast to the linear cursus , Long barrows ,linear cemeteries etc . If enclosure was all that mattered we might expect more rectangularity .
Some of the earlier examples were not only bigger but were also more circular than later examples e.g. Brodgar ,Stonehenge ,Llandegai , Flagstones etc. There are exceptions like Castlerigg and Long Meg but generally the smaller and usually later circles ,despite being easier to mark out were if anything less circular .