In this case, the x-axis is a different unit from the unit of data measured by the radius, so the fact that it's off by a constant multiple doesn't matter; it's

In this case, the x-axis is a different unit from the unit of data measured by the radius, so the fact that it's off by a constant multiple doesn't matter; it's just an arbitrary scale. If both the x- and y-axes were encoding data in the same unit as the area encoded, then it might make sense to correct for the area, but I don't think I've ever seen a plot like that.