I don't think the radii of the circles actually correspond to a particular land area. How would one measure the "extent" of an earthquake, anyway? It's not like they have well-defined, circular bounds.
More likely, the circles are just sized in proportion to the magnitude of the quake, to give you a visual sense of scale.
You can move the map without zooming and the circles follow.
I think the circles are quite proportional. I have a friend of a friend in Osaka who said they did feel the 9.0M one all the way down in Osaka so that circle is correct.
Edit: Actually I had zoomed in a level. I think zooming in one level is a little more accurate. I really don't think people in Korea or Eastern Russia felt it.
More likely, the circles are just sized in proportion to the magnitude of the quake, to give you a visual sense of scale.