Created attachment 2826
Screenshot says it all. When placed in a vertical panel, each button has a row to itself, occupying the entire width of the panel, and then scaling the height to match. It would make much more sense to put all of the buttons in one row.
(I don't see a place to file bugs for this particular plugin? O.o bug #2962 is assigned to Xfce4-xmms-plugin, so I'm going by that.)
Yes, there is no bugzilla component for this plugin.
This is not a bug, it's a feature request, which i probably won't implement. Did you think about the people that were using non-wide vertical panels ? Try setting the width to < 30px, and tell me how to you fit 4 buttons in a row.. Scaling of buttons is done using the panel size, no matter what is the orientation.
I would consider patches implementing it in a sane way though, like selecting orientation in the configuration dialog, regardless of panel orientation, and depending of having a wide enough panel.