The difference is coming most probably because of the resolution settings. See enum QPrinter::PrinterMode
Also, QPrintSupport Class documentation states
Note: When rendering text on a QPrinter device, it is important to realize that the size of text, when specified in points, is independent of the resolution specified for the device itself. Therefore, it may be useful to specify the font size in pixels when combining text with graphics to ensure that their relative sizes are what you expect.