Lengthy lines, when you put the circle through the line it makes it look smaller. Your brain sees separate lines so it makes the line look smaller than the line where the circle isn't going through the line. Also the circle that doesn't go through the line in general makes the line look longer because the circle gives it make length.
DiSpezio, Michael A. Optical Illusion Magic: Visual Tricks & Amusements. New York: Sterling Pub., 1999. Print.