How were the circles drawn?

[N._N.]

Maybe I am just stating the obvious, but a long-established way to draw round shapes is to place a small nail in the center and bind a pencil, pen, piece of chalk etc. with a cord to it. With some adaptations (two nails) you can also use it for an ellipse: You are not allowed to view links. Register or Login to view. 
This should work with quills as well and might also explain mistakes where the circle is larger than it should be, because by pulling the pencil etc. to the outside, you always risk pulling the nails out the center, more so than with a compass from my experience. 

Just as a remark: 'compass' for the drawing tool is somewhat irritating to German speakers because 'Kompass' is only used for the navigational instrument - the tool is a 'Zirkel'. I just realized that apparently most romance languages have the same double meaning for the word as English, so there is nothing lost in translation in that case I guess.