I had similar problem with bigger icons with scummvm and did not find sufficient documentation and filed bug https://bugs.maemo.org/show_bug.cgi?id=3619

In ScummVM the issue was that OS2008 system for some reason preferred small icon in /usr/share/icons over the big ones in other icon folders. Sadly OS2006 did not support small icon in those new folders so my final solution was to keep both and remove /usr/share/icons/scummvm.xpm in postinstall script for OS2008.

Also there is icon cache so without flushing/updating the cache via gtk-update-icon-cache the final result varies.