How it works (from fatfonts.org):
Fatfonts are designed so that the amount of dark pixels in a numeral character is proportional to the number it represents. For example, “2″ has twice the ink than “1″, “8″ has two times the amount of dark ink than “4″ etc. You can see this easily in the set of characters below:
Source for the above and animated: http://visual.ly/fatfonts-player?view=true
My source: Jacob Aron, Making numbers punch their weight, New Scientist, 5 May 2012, 214, 2863, p.12.