Actually that's misleading. If the radius of the circles go up linearly, the area/size of the circle goes up exponentially. So on the logarithmic scale, the height changes with the log of the value but the size goes up exponentially!
Nope. As @arscan says, in this as in most bubble charts, the value determines the circle's area, not the radius. Otherwise it certainly would be misleading.
Edit: nevermind this, it's wrong.