initial size of output pane

I can't remember (if I ever knew) how to get a simple interface where an 
output pane stretches to fit its master.  

I expected the code below to start out at 640x480, and to be resizable to 
larger and smaller than 640x480.

What appears to happen (on LWL) is that the frame is 640x480, but the output 
pane is very tiny - until the first window resizing, at which point the 
output pane snaps to fit the interface.

What am I doing wrong?

(capi:define-interface test-interface ()
  ()
  (:panes
   (opengl capi:output-pane))
  (:layouts 
   (main capi:column-layout '(opengl) :default t))
  (:default-initargs
   :width 640
   :height 480))

pt