Autostart applications list may show too big icons. One example I have - pnmixer, which i added by simply copying desktop launcher from applications to my autostart dir.
Fixed in git master: https://git.xfce.org/xfce/xfce4-session/commit/?id=87f5537d3257459ca68373ef8a98a6d3166a4539 Will be available in next release.