#include <QtWidgets>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QPixmap iconPix(50, 50);
    iconPix.fill(Qt::transparent);
    QPainter painter(&iconPix);
    painter.setBrush(Qt::red);
    painter.setPen(Qt::transparent);
    painter.setRenderHint(QPainter::Antialiasing);
    painter.drawEllipse(iconPix.rect());
    iconPix.setDevicePixelRatio(2);

    QIcon icon;
    iconPix.setDevicePixelRatio(2);
    icon.addPixmap(iconPix);

    QWidget w;
    auto layout = new QVBoxLayout;
    w.setLayout(layout);

    layout->addWidget(new QLabel("Icon pixmap: 50x50@2X"));

    auto btn = new QToolButton;
    btn->setToolButtonStyle(Qt::ToolButtonTextUnderIcon);
    btn->setIcon(icon);
    btn->setIconSize(QSize(64,64));
    btn->setText("Icon size: 64x64");
    layout->addWidget(btn);

    btn = new QToolButton;
    btn->setToolButtonStyle(Qt::ToolButtonTextUnderIcon);
    btn->setIcon(icon);
    btn->setIconSize(QSize(32,32));
    btn->setText("Icon size: 32x32");
    layout->addWidget(btn);

    w.show();

    return a.exec();
}