diff --git a/src/univ/themes/gtk.cpp b/src/univ/themes/gtk.cpp
index 2f383b594c..102560c637 100644
--- a/src/univ/themes/gtk.cpp
+++ b/src/univ/themes/gtk.cpp
@@ -2267,12 +2267,13 @@ void wxGTKRenderer::AdjustSize(wxSize *size, const wxWindow *window)
 #if wxUSE_SCROLLBAR
     if ( wxDynamicCast(window, wxScrollBar) )
     {
-        // we only set the width of vert scrollbars and height of the
-        // horizontal ones
-        if ( window->GetWindowStyle() & wxSB_HORIZONTAL )
-            size->y = m_sizeScrollbarArrow.x;
-        else
-            size->x = m_sizeScrollbarArrow.x;
+        /*
+        Don't adjust the size for a scrollbar as its DoGetBestClientSize
+        already has the correct size set. Any size changes here would get
+        added to the best size, making the scrollbar larger.
+        Also skip border width adjustments, they don't make sense for us.
+        */
+        return;
     }
     else
 #endif // wxUSE_SCROLLBAR
diff --git a/src/univ/themes/win32.cpp b/src/univ/themes/win32.cpp
index 0738111495..7150c7a98b 100644
--- a/src/univ/themes/win32.cpp
+++ b/src/univ/themes/win32.cpp
@@ -3142,14 +3142,12 @@ void wxWin32Renderer::AdjustSize(wxSize *size, const wxWindow *window)
 #if wxUSE_SCROLLBAR
     if ( wxDynamicCast(window, wxScrollBar) )
     {
-        // we only set the width of vert scrollbars and height of the
-        // horizontal ones
-        if ( window->GetWindowStyle() & wxSB_HORIZONTAL )
-            size->y = m_sizeScrollbarArrow.y;
-        else
-            size->x = m_sizeScrollbarArrow.x;
-
-        // skip border width adjustments, they don't make sense for us
+        /*
+        Don't adjust the size for a scrollbar as its DoGetBestClientSize
+        already has the correct size set. Any size changes here would get
+        added to the best size, making the scrollbar larger.
+        Also skip border width adjustments, they don't make sense for us.
+        */
         return;
     }
 #endif // wxUSE_SCROLLBAR