First of all, i prefer the white background / bigger screen.
Second to mention that this happens in Maemo AND Easy Debian.

My oppinions:
I think it should be possible to use always the preferred display mode or even to switch between them. Zooming into a window like in the browser would be nice for such gtk+-apps, where you need more display space. Also a tool for grouping windows into one would be a nice-to-have (especially for Gimp and Dia).