Thanks for the bug report. There was an error in the scroll wheel event handler that stepped the brush radius from 1px to 3px, without the intervening 2px step. I have fixed this in the next developmental build, and I will post back here when it's available (some time between today and next weekend; sorry I can't be more precise!)
The broader problem is that the brush radius is stored as an integer, which noticeably limits the brush radius options at small brush sizes. A better way would have been for me to use a floating point number for the brush radius. This would, for instance, allow a brush width of 3px using a radius of 1.5. However that decision was made some time ago and cannot be quickly undone. Nonetheless, if the improved brush size option in the next build (i.e. 2px radius) is not adequate for your needs, I will pop a floating point brush radius on my to-do list.