If you build and run the tabdialog example experiment (/qtbase/examples/widgets/dialogs/tabdialog) in Qt5 and then try to resize the dialog, it's possible to resize the dialog to be about 1 pixel tall and 60 pixels wide.
Using the exact same code compiled using Qt 4.8.4, it's not possible to resize the dialog smaller than the size of the contents of the dialog.
In the attached screenshot, TabDialog_Qt4_8_4.png has been resized as small as it will allow me to resize it. The dialog in TabDialog_Qt5_1_beta.png could be resized even smaller than what I show, which is clearly too small to properly display the contents.
This behavior is not specific to the tabdialog example, as I see the same thing in all dialogs in my application, which are all based on a QDialog subclass.
I had to change line 41 of tabdialog.cpp to "#include <QtGui>" to get the example to compile using Qt 4.8.