If you draw a circle, the smaller you draw it, the more it seems to look like a dot. The larger you draw the circle, the more the curve is so spread out, that when you focus on one part of that huge circle, it begins to look like a long line with a slight curve. Keep making it larger and the curve seems to be less and less noticeable.

You can also see this effect by holding a ball and moving it closer or further away from your eyes while focusing on the curve at the edge.