In QMacStyle, any QPushButton that is in a layout will have space below it, regardless or whether or not that layout's spacing and margin is set to zero.
This is not the case with all widgets. For example, this problem is not apparent with QToolButtons.
This problem is most apparent when the pushbuttons are hidden. When the buttons are hidden, the space still exists.
This bug does not occur in other styles.
Note from development:
This is a bug but is workaroundable. It is a result of the layout rect. See: http://doc.trolltech.com/qq/qq23-layouts.html
You can workaround this by setting the Qt::WA_LayoutUsesWidgetRect attribute on the widgets that cause problems.