What is the physical size of a single pixel?
On a 15 inch 1366×768 px laptop screen, a single pixel should be about 0,2341 mm wide.
On a 5 inch 1280×720 px phone screen, a single pixel should be about 0,0864 mm wide. But here comes that Supporting Multiple Screens article on Android Developers website that says that device independent pixel size on Android can be calculated from the following equation:
px = dp * (dpi / 160)
On the mentioned mobile screen, a device independent pixel is approximately 1,8358 times bigger than the physical one: 0,1586 mm. That’s about 68% of the laptop’s (desktop’s) pixel size — on mobile, your pixels are already smaller.
To answer your question: I think you shouldn’t make base font size on mobile different than it is on desktop, because the browser (or the OS) already takes care of it. However, depending on your overall design and the content, you may find it useful to make headers a bit smaller on mobile because of narrower screen.