This is not a bug but more a feature. If your screen resolution is big enough - more than 1024x768 - Nero Linux main windows will have a minimum size, to make sure that every single widget can be displayed without a problem.
Of course, with netbooks, the screen resolution gets smaller, but Nero Linux is clever enough to detect that. If you launch the application on such a device, you will - for example - not see any labels on the toolbar buttons. Also the dialog margins and spacings might be trimed down to fit to the resolution.