According to Jeff Mayo in his book 'the planets and human behaviour' p 27, the esoteric meaning of the circle is 'representative of wholeness,
completeness, the life-giving principle or divine spirit.'
AGS Norris in Trancendental Astrology p52 says of the Ego 'the true spiritual part of man [is] the point, in a rhythm of circles, [that] gradually
extends its radius of action towards the priphery and builds within the circle its own Star, [and] so [then] does the Ego's conciousness become merged
in the Divine Consciousness sybolised by the circle''
This second quote refers to the point in the circle the symbol for the sun and not just the circle, I think this point is an important part of the
symbol coming to mean 'whole'.
The wholeness of circle is not complete, because pi is an irrational number; there fore when you compute the area or circumference of a circle it is
never quite complete. The wholeness is only an illusion of our minds.