How much room a small control reserves for the finger that works it.
A switch paints a track 32 pixels tall; a checkbox paints a box of 20. Neither is
something a finger can be asked to hit. The reference lays both out inside a
48-pixel square and paints the small thing in the middle of it, and it makes that a
theme-wide setting with a per-widget override, because it is the kind of decision an
application makes once (theme_data.dart:172).
This is a layout answer, not a visual one. What the control paints does not
change; the room around it does, and so does the area a click may land in.