All of your examples look like the windows have not been set to full screen. This explains why they don't "fit on the screen" as the window is bigger than the display. Pretty much all windows use full screen mode on windows mobile since there is not enough room for lots of windows.
The last example looks like it is set to full screen but the text information and graphic elements are to big for the screen size.
So on the whole it looks like Qt is working as best that it can with layouts that are meant for the desktop. Given the size difference between deskstop resolutions and device resolutions, desktop top window layouts will almost never work on small device screen. So any porting of a Qt application written for the desktop size resolutions will need the GUI relayed out and the input methods redone as both are vastly different between desktops and mobile devices.
Your idea of "compile once and deploy everywhere" doesn't make sense. The link you provided is dealing with the MAC OS and the versions of MAC OS.
I would think one of the main benifits of a cross-platform GUI library like Qt would be the one set of source code that can be compiled without change between lots of different platforms. Across desktop platforms that makes sense. Across different device platforms that "kind-of" makes sense, altho input methods can be vastly different between devices and also main GUI concepts can also be vastly different (compare WM device to a iPhone device for example) as well.
So think Qt could work for you but you have to:
- Aways use full screen mode.
- Provide layouts and graphic elements that since for the screen size.
One problem you may have if you wish to support multiple devices, is supporting multiple screen sizes. Also supporting dynamically change screen sizes between protrait and landscape. These sorts of requirements are part of Mobile2Market Certificate requirements and part of the new upcomming Microsoft Mobile Marketplace (altho I can't find the like to the requirements).