Run Swing Set. Change L&F to Motif. Select the Internal Frame tab. Create an iconifiable internal frame using the Frame Creator. Iconify the internal frame you created by clicking on the iconify button, then deiconify it by double-clicking on the icon.

Now change the L&F back to Java L&F and iconify the internal frame. The icon will have the Motif size in the Metal look.

There are also various positioning problems with icons when changing L&F.