Man, this visualization bothers me.
We're clearly seeing how often various kinds of pitches are thrown compared to each other, but what visual element is proportional to the number of pitches of that type?
Is it the radius of the circle? Lots of graph-makers make that mistake, and this guy has already made it multiple times with other graphs (maybe I'll bring them up later).
Is it the area of the circle (proportional to the radius squared)? That's usually what these things should mean, although I generally dislike denoting values with circle sizes because they're harder to compare visually (it's trivial to see that a bar is exactly twice the size of another one, but it's hard to see that a circle is exactly twice the size of another one), plus I never know whether to trust that the designer wants me to measure in 2D vs 1D.
But wait, that's not all! The circles are shaded to look like spheres! So maybe I should be comparing based on the radius cubed!
The sad thing is that I know that this designer is a big Tufte fan.