The size() reports the size without the unifiedtoolbar, but calling resize resizes the whole window, including the toolbar.

http://doc.trolltech.com/4.7/qmainwindow.html#unifiedTitleAndToolBarOnMac-prop

//in any QMainWindow on mac with unified toolbar, for example the macmainwindow demo application.
resize(500,500);
qDebug() << "size" << size();
//output:
size QSize(500, 462)

This is constant, so an easy workaround is to adjust the returned height value depending on the height of the toolbar.