buttonPaddingHorizontal

PhoneButtonBase's Padding="10,3,10,5" — the button's content inset, asymmetric top to bottom.

It lives here rather than beside the button because it has to be scaled by scaled with everything else. Left as a constant in the component it stayed at 7.5/2.25/3.75 dp while the type ramp grew with the metric set, and a scaled-up button came out with its text pressed against the frame.