I've checked this; alas the issue couldn't be fixed as it strictly depends on the browsers architecture which no script could control.
Obviously each browser has its own pre-defined minimum width of the pop-up window it could handle, this width is reserved for various browser tools and elements (such as window title, Minimize/Expand/Close buttons, address bar and its plugins and so on). In Opera it looks mostly the same:
As soon as the content (image) width doesn't exceed this defined minimum, the space is added to handle the minimum.
To work-around this in a native way, you could edit addon_fileupload_options.php
and make default values of a thumbnail larger, for example set them as:
$maxThWidth=300; //thumbnail width
$maxThHeight=300; //thumbnail height
also adjust the $autoGallery
setting (set it to 3 or 2 by default, meaning how many pictures would fit in a row).
This will make images having smaller size not being opened in a pop-up, but straight visible on the page.