Disable margin when using QMdiArea

At the moment, I am converting my program from using QWorkspace to QMdiArea. But I have the problem. Because I am at home, and I don't have Qt. So I use 2 other programs to show the difference.

Please see the window frame:

QMdiArea:
http://img524.imageshack.us/img524/5192/marginzg3.jpg

QWorkspace:
http://img297.imageshack.us/img297/7895/nomarginah4.jpg

Please help me how to remove margin in QMdiArea. Thanks.

This margin might be introduced by the layout, not MDI area.