The DirectWrite font database seems to be storing more memory per resolved font than the GDI engine. For an application that resolves all fonts (e.g. to show a font combo box) this can cause a noticeable increase in memory footprint.

The following loop to resolve all fonts on the system causes a memory footprint of 28 MB for me with DirectWrite, versus 16 MB with GDI:

    for (int i = 0; i < QFontDatabase::families().size(); ++i) {
        QFontInfo fi(QFont(QFontDatabase::families().at(i)));
        fi.family();
    }