Hopefully picture will explain the problem... ??? The thumbnail is just so tiny! Is there anyway to make it the size of the larger thumbail? I thought you changed it in config "Max dimension of a thumbnail **" but that was for the large thumbnail.
(https://coppermine-gallery.com/forum/proxy.php?request=http%3A%2F%2Fimg122.echo.cx%2Fimg122%2F8441%2Fimage16gx.jpg&hash=20e0a178ea23dd4eff883fc8577c4414e845a232) (http://www.imageshack.us)
In the config, modify the "Size of thumbnails in pixels" and make it larger.