QFontMetrics::boundingRect size decreases when string length increases

#include <QtDebug>
#include <QtGui>

int main(int argc, char *argv[])
{
  QApplication a(argc, argv);
  QFontMetrics metrics(QFont("Arial", 11));
  qDebug() << metrics.boundingRect("test");
  qDebug() << metrics.boundingRect("test ");
  return a.exec();
}

On my machine, the output is:
QRect(-1,-14 26x18)
QRect(-1,-14 22x18)

Note that size decreases when the space is added. I could understand it being the same (the space doesn't need any painting room), but decrease? This is causing problem if this bounding rect is used for QPainter::drawText, because the smaller rectangle clips the text.