The Silverstone is a bit reluctant to show the external PSU - certainly there's no mention of its size. I assume it would be very similar to a 120W laptop PSU.
Found only one review of it (in German and not very detailed):
http://www.minitechnet.de/silverstone_lc12_1.html
Still at 184x93x255mm it's only 4.36 litres which is pretty small. However, the Mac Mini measures 165x165c51mm which is less than 1.4 litres making it less than a third of the size of the Silverstone.
Of course a self-built ITX machine is upgradeable whereas the Mac Mini is barely able to be opened.