Screenshot from Commons:
This is consistent across all TagMultiselect and MenuTagMultiselect UIs on Commons and seems to be from the OOUI 0.41.0 release, as it isn't present yet on Wikipedia.
Possibly related to this CSS rule:
.oo-ui-buttonElement-frameless.oo-ui-iconElement > .oo-ui-buttonElement-button {
min-width: 32px;
min-height: 32px;
}... which used to be a calculated em value rather than pixels.
Conformed in Chrome and Firefox.
