Size of widgets in QTabWidget is 640x480 before shown the first time

#include <QApplication>
#include <QString>
#include <QTabWidget>
#include <QWidget>
#include <QTimer>
#include <QDebug>

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    QWidget w1;
    QWidget w2;

    QTabWidget tw;
    tw.show();
    tw.addTab(&w1, QString("widget 1"));
    tw.addTab(&w2, QString("widget 2"));

    QTimer timer;
    timer.setInterval(1000);
    QObject::connect(&timer, &QTimer::timeout, [&w1, &w2]() {
            qDebug() << w1.size();
            qDebug() << w2.size();
    });
    timer.start();

    app.exec();
    return 0;
}