QText, how to only change font family?

[UriTK]

Hello. Sorry for the stupid question, but I've written up some code that I expected to only change font family, but it also changes the font size.

int id = QFontDatabase::addApplicationFont(font_path);
    if(id != -1)
    {
       QStringList family = QFontDatabase::applicationFontFamilies(id);
       ui_vp_message->setFont(family.at(0));
    }

int id = QFontDatabase::addApplicationFont(font_path);
    if(id != -1)
    {
       QStringList family = QFontDatabase::applicationFontFamilies(id);
       ui_vp_message->setFont(family.at(0));
    }

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I have tried specifically using setFontFamily, but for some reason this does not change the font at all.
Am I approaching this the wrong way? Is there some other way I could change the font family?