I still don't see how a tire spinning faster means it flings more stuff.

I can imagine that the radius of the tire affects the angle at which stuff is flung and maybe a smaller wheel flings stuff at a worse angle. I can imagine that fatter tires fling more stuff than skinnier tires. And I can imagine that on a bike with smaller wheels, the bottom bracket is closer to the ground than a bike with bigger wheels which could mean that somebody gets more stuff on them.

But I don't see how rotational speed makes any difference.

And if the brake pads are the same size, they should wear at the same rate (unless the rims are significantly hotter because they are smaller and therefore cause the pads to wear faster). Smaller rims will wear faster than larger ones.