Font size calculation when painting in a QImage

[Ishark]

Hello,
I'm trying to generate a modified image starting from a preexisting PNG file and drawing some text on it. I'm not getting what I think I should get
In particular, I have some issues with font sizes.
I have found that the PNG resolution plays a role in the calculation of the actual pixel size of the font, if I increase the resolution and keep the font size fixed, the actual rendered text is smaller. The sizes do not match what I see if I do the same in inkscape, but this is not a problem. My problem is that there seems to be a "minimum pixel size" which limits the size of the smallest font I can draw. Right now I have the PNG at 72x72 dpi and a font size of 2 looks identical to font size 10.

The code:

int main(int argc, char **argv)
{
    QApplication a(argc, argv);
    QImage* card = new QImage("blank72.png");
    QPainter* painter = new QPainter(card);
	QFont* FontText = new QFont("Serif", 2, QFont::Normal);
	QFont* FontText2 = new QFont("Serif", 10, QFont::Normal);
	painter->setRenderHint(QPainter::TextAntialiasing, true);
    painter->setPen(Qt::black);
	painter->setFont(*FontText);
	painter->drawText(QRect(0,0,256,128), Qt::TextWordWrap, "Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt");
	painter->setFont(*FontText2);
	painter->drawText(QRect(0,128,256,128), Qt::TextWordWrap, "Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt");
    QLabel* label = new QLabel(0);
    label->setPixmap(QPixmap::fromImage(*card));
    label->show();
 
    return a.exec();
}

int main(int argc, char **argv)
{
    QApplication a(argc, argv);
    QImage* card = new QImage("blank72.png");
    QPainter* painter = new QPainter(card);
	QFont* FontText = new QFont("Serif", 2, QFont::Normal);
	QFont* FontText2 = new QFont("Serif", 10, QFont::Normal);
	painter->setRenderHint(QPainter::TextAntialiasing, true);
    painter->setPen(Qt::black);
	painter->setFont(*FontText);
	painter->drawText(QRect(0,0,256,128), Qt::TextWordWrap, "Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt");
	painter->setFont(*FontText2);
	painter->drawText(QRect(0,128,256,128), Qt::TextWordWrap, "Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt");
    QLabel* label = new QLabel(0);
    label->setPixmap(QPixmap::fromImage(*card));
    label->show();

    return a.exec();
}

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blank72.png is a 256x256 white image with 72dpi resolution.

Side question: is there any easy way to have the text justified?