Why does Qt have problems handling small QGraphicsItems - c++

Question

When I define several QGraphicsitems with small dimensions e.g. transforming metric lines directly into QGraphicLineItems with the same length (e.g few Meters) the whole redering system becomes very slow and even is prone to crashes. On the other hand, when the items are "bigger" (length > 100) no problems arise.
To give a more concrete example: I use the following code to transform QGraphicItems into a pixmap.
QGraphicsPixmapItem* transformToPixmapItem(std::vector<QGraphicsItem>& items,int maxRes){
QRectF boundingRect;
boostForeach(QGraphicsItem* pItem,items) {
boundingRect = boundingRect.united(pItem->boundingRect());
}
QSize size(boundingRect.size().toSize());
double const scale = std::min(16.0,double(maxRes)/boundingRect.size().width());
QPixmap pixmap(size*scale);
pixmap.fill(Qt::transparent);
QPainter p(&pixmap);
//p.setCompositionMode( QPainter::CompositionMode_Source );
p.translate(-boundingRect.topLeft()*scale);
p.scale(scale,scale);
QStyleOptionGraphicsItem opt;
boostForeach(QGraphicsItem* item,items) {
item->paint(&p, &opt, 0);
}
p.end();
deleteVector(items);
QGraphicsPixmapItem* item = new QGraphicsPixmapItem(pixmap);
item->setScale(1.0/scale);
item->setOffset(boundingRect.topLeft()*scale);
qDebug() << "Pixmap done. 1/Scale " << 1.0/scale;
return item;
}
On "bigger" items this works fine on small items it crashes, the condition seems to be 1/scale. If this value used to downsize the resulting pixmap item gets too small the function completes but the rendering of the resulting item crashes.
Besides this, as already stated the rendering of objects whose physical size has been augmented e.g. by factor 100 seems to be much faster than small objects.
Is there some general guide line on recommended Graphic dimensions and where does this effect come from ?