use Less' extend feature in btn-group-lg etc.

dist/css/bootstrap.css

@@ -2311,19 +2311,22 @@ fieldset[disabled] .btn-link:focus {
   color: #999;
   text-decoration: none;
 }
-.btn-lg {
+.btn-lg,
+.btn-group-lg > .btn {
   padding: 10px 16px;
   font-size: 18px;
   line-height: 1.33;
   border-radius: 6px;
 }
-.btn-sm {
+.btn-sm,
+.btn-group-sm > .btn {
   padding: 5px 10px;
   font-size: 12px;
   line-height: 1.5;
   border-radius: 3px;
 }
-.btn-xs {
+.btn-xs,
+.btn-group-xs > .btn {
   padding: 1px 5px;
   font-size: 12px;
   line-height: 1.5;
@@ -3196,24 +3199,6 @@ input[type="button"].btn-block {
 .btn-group.open .dropdown-toggle {
   outline: 0;
 }
-.btn-group-xs > .btn {
-  padding: 1px 5px;
-  font-size: 12px;
-  line-height: 1.5;
-  border-radius: 3px;
-}
-.btn-group-sm > .btn {
-  padding: 5px 10px;
-  font-size: 12px;
-  line-height: 1.5;
-  border-radius: 3px;
-}
-.btn-group-lg > .btn {
-  padding: 10px 16px;
-  font-size: 18px;
-  line-height: 1.33;
-  border-radius: 6px;
-}
 .btn-group > .btn + .dropdown-toggle {
   padding-right: 8px;
   padding-left: 8px;

less/button-groups.less

@@ -96,9 +96,9 @@
 //
 // Remix the default button sizing classes into new ones for easier manipulation.
 
-.btn-group-xs > .btn { .btn-xs(); }
-.btn-group-sm > .btn { .btn-sm(); }
-.btn-group-lg > .btn { .btn-lg(); }
+.btn-group-xs > .btn { &:extend(.btn-xs); }
+.btn-group-sm > .btn { &:extend(.btn-sm); }
+.btn-group-lg > .btn { &:extend(.btn-lg); }
 
 
 // Split button dropdowns