I'm using the newest C4D version on macOS. This problem exists already for a long time. There has been several versions of Cinema and macOS, so I doubt it's a driver problem.
I have set a shortcut for the driving the size of the Magnet brush as well, I can control the Magnet and Sculpt brush with the same key and mouse click. The magnet brush always is on pint, I can change the size by the value of one very smoothly. But as soon as I change to a sculpt brush it's getting very uncontrollable and the change happens at least with the value of 4. Tested that On a simple sphere, so it definitely not any performance issue.
Also happens with Middle click as well, exactly the same.