Changing this values should only support horizontal movement as we are no longer trying to match the size of the cursor and the size of the circle preview in the widget.
I still find changing the brush size to small problematic.
You have to move towards this specific point on the left side of the brush, it's very sensitive there, and if you overshoot the brush becomes bigger again.
Maybe it makes more sense to never make the brush bigger when moving to the left, and give the user e.g. 200 pixels to the left of the brush center to tweak the cursor size from 0 to the current radius?
I think this is an improvement.
Still recommend to find a way to make setting a small radius less finicky. For example a difference in brush with of 2 and 4 pixels is significant, but moving your mouse exactly 2 pixels to make it use one or the other is hard.
It's the same in 2.79 though.