Bootstrap class: .btn-group-sm

<div class="btn-group btn-group-sm" role="group" aria-label="Basic example">
  <button type="button" class="btn btn-secondary">Left</button>
  <button type="button" class="btn btn-secondary">Middle</button>
  <button type="button" class="btn btn-secondary">Right</button>
</div>

Preview

Sass source

/* _button-group.scss:67 */
.btn-group-sm > .btn { @extend .btn-sm; }
.btn-group-lg > .btn { @extend .btn-lg; }


//
// Split button dropdowns
//

.dropdown-toggle-split {
  padding-right: $btn-padding-x * .75;
  padding-left: $btn-padding-x * .75;

  &::after,
  .dropup &::after,
  .dropright &::after {
    margin-left: 0;
  }

  .dropleft &::before {
    margin-right: 0;
  }
}

Uses variables

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/* _button-group.scss:67 */
$btn-padding-x
$btn-padding-x

More in Bootstrap Button Groups