figsize has no effect in embedded GTK2


I tried to embed matplotlib in a GTK2 application (using pygtk & glade) but encountered a problem.
As I have been playing with it for a while I think that for its demonstration it is enough to look at the matplotlib example: (from the example directory of matplotlib)

There, for me the figsize line:
fig = Figure(figsize=(6,6), dpi=100)
has no effect whatsoever (when tryong to change the size)…
The window size is simply set with
and the figure is always of the same size…
Can anybody tell me why?

(In my own application, if I do not explicitely specify a window size, I always get a window that is too small and has to be resized by hand.)

Currently I am using Windows XP with
matplotlib 0.87.4
pygtk 2.8.6

Thank you for any idea!