#include <QtWidgets>

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    QIcon::setThemeName("Adwaita");
    QIcon icon = QIcon::fromTheme(QStringLiteral("folder-documents"));
    qDebug() << icon.availableSizes(); // QList(QSize(16, 16), QSize(128, 128))
    QPixmap pixmapRegular = icon.pixmap(QSize(128, 128), 1.0);
    qDebug() << pixmapRegular.size(); // QSize(128, 128): OK
    QPixmap pixmapScaled = icon.pixmap(QSize(128, 128), 2.0);
    qDebug() << pixmapScaled.size(); //  QSize(16, 16): wrong
}