QMdiSubWindow's size decrease when deactivate

A QMdiArea has two QMidSubWindow, which of them are both showMaximized(), however, I found the size of the deactivate one is the normal size.
code is as below:
int main(int argc, char *argv[])
{
QApplication a(argc, argv);

QMdiArea* pArea = new QMdiArea();
QMdiSubWindow* pW1 = new QMdiSubWindow();
QMdiSubWindow* pW2 = new QMdiSubWindow();

pArea->show();
pArea->addSubWindow(pW1);
pArea->addSubWindow(pW2);
pArea->resize(500,500);
pW1->showMaximized();
pW2->showMaximized();

qDebug() << "W1:" << pW1->size(); // output 139,37
qDebug() << "W2:" << pW2->size(); // output 500,500

return a.exec();
}

I saw the source code and I found that a maximized QMdiSubWindow would do shorNormal() when it become deactivated, but I can't get it. Why?