Not sure about circles as sole UI primitives, but this is a great start that could grow to support rectilinear views as well.

The big thing I would take a look at is the animation: Ease-in with a hard stop is not a pleasant motion for a zooming UI. I'd try an ease-in-out, and see if you can synchronize the scaling and the positioning so the new circle smoothly zooms into place rather than zooming offscreen and jumping into the center at the end of the animation.

Keep up the good work!