Turns out only MSW has odd behavior for bitmap sizing

common/widgets/bitmap_button.cpp

@@ -95,7 +95,7 @@ void BITMAP_BUTTON::SetPadding( int aPadding )
 void BITMAP_BUTTON::SetBitmap( const wxBitmapBundle& aBmp )
 {
     m_normalBitmap = aBmp;
-#ifdef __WXMAC__
+#ifndef __WXMSW__
     m_unadjustedMinSize = m_normalBitmap.GetDefaultSize();
 #else
     m_unadjustedMinSize = m_normalBitmap.GetPreferredBitmapSizeFor( this );
@@ -369,4 +369,4 @@ bool BITMAP_BUTTON::IsChecked() const
     wxASSERT_MSG( hasFlag( wxCONTROL_CHECKABLE ), wxS( "Button is not a checkButton." ) );
 
     return hasFlag( wxCONTROL_CHECKED );
-}
+}