See http://ensc.de/xfce4/pager-xfce4.png There are 10 desktops having 2 viewport each. The viewports are having a dimension of 1600x1200 (4:3) but pager shows them with an aspect ratio of 2:3. E.g. the applications in desktop #3, #5 (first viewports) and #10 (both viewports) are fullscreen and are having a size of 1600x1200.
I think this is fixed in the libwnck pager. Please reopen of you still have this probalem with trunk.