When an empty icon is added to systray (i.e. thunars file operation) or gnome-bluetooth on gentoo (w/o iconset) all shown icons became smaller. For example on panel size 30 icons are about 24 (just guess), when an empty icon is added to tray, all visible icons became about 16px. It looks like systray calculates space required for displaying icons in invalid way. Occurs on two different Gentoo x86_64 machines.
Created attachment 3256 Screenshot describing problem Systray i set to 1 row, frameless, no hidden icons. I have been using thunar with file operations and I believe that empty space comes from its "progress dialog" icon. But this problem occurs with other iconless applications as well.
This might be a duplicate of http://bugzilla.xfce.org/show_bug.cgi?id=6892 , which is also about the shrinking icons in the panel.
Should be fixed in ad5a85c (xfce4-panel-4.7.7 once released) please reopen if the bug is still reproducible with this panel.
Snapshots for testing: http://foo-projects.org/~nick/packages/xfce4-panel-4.7.6git-c37dd16.tar.bz2 or wait for xfce4-panel-4.7.7.