Digital Light & Color

When using a TouchPad, a zoom gesture controls brush width.
When shift is pressed, a zoom gesture controls skew
When Ctrl is pressed, a zoom gesture also controls brush width.

Suggestion : a zoom gesture should be defined to control brush size. For example, when neither Ctrl, nor Alt , nor Shift are pressed, a zoom gesture could control brush size.

The interpretation of touchpad gestures is done by Windows, not PWP. Not sure why it would work that way -- maybe check your touchpad settings.