MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    QPushButton *btn = new QPushButton("TEST");
    connect(btn, &QPushButton::clicked, [=](bool checked ) {
        qDebug() << "Button size: " << btn->size();

        QPixmap defPix = btn->grab();
        qDebug() << "Default grab. Pixmap size: " << defPix.size();
        defPix.save("defpix.png");

        QPixmap dprPix = btn->grab(QRect(0,0, btn->width() * btn->devicePixelRatio(), btn->height() * btn->devicePixelRatio()));
        qDebug() << "Grab using dpr. Pixmap size: " << dprPix.size();
        dprPix.save("dprpix.png");
     });
    ui->verticalLayout->addWidget(btn);
}

Button size:  QSize(378, 17)
Default grab. Pixmap size:  QSize(378, 17)
Grab using dpr. Pixmap size:  QSize(756, 34)