In brief
outline-width sets outline thickness. Increasing it widens the indication but does not automatically enlarge the button's interactive box.
Useful to know
outline-width sets outline thickness. Increasing it widens the indication but does not automatically enlarge the button's interactive box. [1]
For easier activation, inspect the control's dimensions and padding. For more visible focus, consider thickness, colour and offset together. These are different goals even though both concern usability.
Sources
For further detail and to check the information: