It is normal for the font size to appear smaller on a full hd canvas ( 1920x1080pixels, default resolution of an IntuIFace experience).
In MS Office, for example an A4 paper size at a 72dpi has the rendering size of 824x595 pixels. Should you create an IntuIFace experience with this resolution, you will notice a text with a font size of 9 becomes readable. MS Office automatically adapts the size of the canvas to the resolution of your screen, thus the computation of the text in MS office is entirely dependent on the display you are using, When you compare the two font sizes you also have to take in consideration the zoom factor of your canvas in both IntuIFace and MS office. I hope this sheds some light in regards to the font size.