The area a finger must land on to fire a control, kept large even when the icon inside it is small. WCAG asks for at least 24px; platform guidance prefers 44 to 48px.
The glyph is 16px but its touch target is 44px: padding makes the tap forgiving without making the icon loud.
A target the eye finds and the finger keeps missing is its own small cruelty. The fix is to decouple the hit area from the artwork: keep the glyph small, grow the tappable region with padding or a pseudo-element. Crowded toolbars are where this fails most, so the space between targets counts as much as their size.