On second thought, only allocate the extra space when determining the element size; still paint the indicator at its "real" size. Not part of the build.

widget/src/gtk/gtkdrawing.c

@@ -172,13 +172,6 @@ moz_gtk_checkbox_get_metrics(gint* indicator_size, gint* indicator_spacing)
   if (indicator_spacing)
     *indicator_spacing = klass->indicator_spacing;
 #endif
-
-  /* hack alert: many themes don't correctly report the indicator size,
-     and have indicators larger than the default 10px indicator_size.
-     So, leave a little slop room. Too bad there's no way to tell
-     if the theme has overridden the default indicator size.  */
-
-  *indicator_size = MAX(*indicator_size, 14);
 }
 
 void
@@ -194,8 +187,8 @@ moz_gtk_checkbox_paint(GdkWindow* window, GtkStyle* style,
 
   moz_gtk_checkbox_get_metrics(&indicator_size, NULL);
 
-  /* left justified, vertically centered within the rect */
-  x = rect->x;
+  /* centered within the rect */
+  x = rect->x + (rect->width - indicator_size) / 2;
   y = rect->y + (rect->height - indicator_size) / 2;
   width = indicator_size;
   height = indicator_size;

widget/src/gtk/nsNativeThemeGTK.cpp

@@ -519,7 +519,12 @@ nsNativeThemeGTK::GetMinimumWidgetSize(nsIRenderingContext* aContext,
 
       gint indicator_size, indicator_spacing;
       moz_gtk_checkbox_get_metrics(&indicator_size, &indicator_spacing);
-      aResult->width = aResult->height = indicator_size;
+
+      // Hack alert: several themes have indicators larger than the default
+      // 10px size, but don't set the indicator size property.  So, leave
+      // a little slop room by making the minimum size 14px.
+
+      aResult->width = aResult->height = MAX(indicator_size, 14);
       *aIsOverridable = PR_FALSE;
     }
     break;

widget/src/gtk2/nsNativeThemeGTK.cpp

@@ -519,7 +519,12 @@ nsNativeThemeGTK::GetMinimumWidgetSize(nsIRenderingContext* aContext,
 
       gint indicator_size, indicator_spacing;
       moz_gtk_checkbox_get_metrics(&indicator_size, &indicator_spacing);
-      aResult->width = aResult->height = indicator_size;
+
+      // Hack alert: several themes have indicators larger than the default
+      // 10px size, but don't set the indicator size property.  So, leave
+      // a little slop room by making the minimum size 14px.
+
+      aResult->width = aResult->height = MAX(indicator_size, 14);
       *aIsOverridable = PR_FALSE;
     }
     break;