QGraphicsProxyWidget: Scaling of text does not work well on Mac

When scaling a QGraphicsProxyWidget containing widgets with text, then the text does not look crisp on Mac.

Test case main.cpp to reproduce issue
====================================
#include <QtGui>

int main(int argc, char **argv)
{
QApplication app(argc, argv);

QGroupBox *groupBox = new QGroupBox("Contact Details");
QLabel *numberLabel = new QLabel("Telephone number");
QLineEdit *numberEdit = new QLineEdit;

QFormLayout *layout = new QFormLayout;
layout->addRow(numberLabel, numberEdit);
groupBox->setLayout(layout);

QGraphicsScene scene;
QGraphicsProxyWidget *proxy = scene.addWidget(groupBox);
proxy->scale(2, 2);

QGraphicsView view(&scene);
view.show();

return app.exec();
}