Looking at the source code is a good way to answer your question (although J-P has already answered it a couple of times):
Qt Code: 
  1. QRectF QGraphicsRectItem::boundingRect() const
  2. {
  3.     Q_D(const QGraphicsRectItem);
  4.     if (d->boundingRect.isNull()) {
  5.         qreal halfpw = pen().widthF() / 2;
  6.         d->boundingRect = d->rect;
  7.         if (halfpw > 0.0)
  8.             d->boundingRect.adjust(-halfpw, -halfpw, halfpw, halfpw);
  9.     }
  10.     return d->boundingRect;
  11. }
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Especially take a look at line #6.

d->rect is set by QGraphicsRectItem::setRect().