Bootstrap class: .btn-sm

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<button type="button" class="btn btn-primary btn-sm">Small button</button>

Preview

Sass source

/* _button-group.scss:90 */
.btn-sm + .dropdown-toggle-split {
  padding-right: $btn-padding-x-sm * .75;
  padding-left: $btn-padding-x-sm * .75;
}

/* _buttons.scss:117 */
.btn-sm {
  @include button-size($btn-padding-y-sm, $btn-padding-x-sm, $font-size-sm, $btn-line-height-sm, $btn-border-radius-sm);
}

Uses variables

/* _button-group.scss:90 */
$btn-padding-x-sm
$btn-padding-x-sm

/* _buttons.scss:117 */
$btn-padding-y-sm
$btn-padding-x-sm
$font-size-sm
$btn-line-height-sm
$btn-border-radius-sm

More in Bootstrap Button Modifiers