The larger anis, the smaller its radius appears. For example, the three illustrated above belong circles of the same radius.
This entry contributed by Margherita Barile
Carraher, R. G. and Thurston, J. B.New York: Reinhold, 1966.
Barile, Margherita. "Three-Arc Illusion." From MathWorld–A Wolfram Web Resource, created by Eric W. Weisstein. http://mathworld.wolfram.com/Three-ArcIllusion.html