/* change all .btn to .btn-sm size on xs */
@include media-breakpoint-between(xs,sm){
  .btn {
      @include button-size($btn-padding-y-sm,$btn-padding-x-sm,$font-size-sm,$btn-border-radius-sm);
  }
}

https://www.codeply.com/go/FoEUO7XCDU

更新 – Bootstrap 4 Beta
这个按钮大小的mixin略有变化：

@include media-breakpoint-between(xs,sm){
  .btn {
      @include button-size($input-btn-padding-y-sm,$input-btn-padding-x-sm,$line-height-sm,$btn-border-radius-sm);
  }
}