Is it really the objective to make a tiny mainboard? I think “how small can we go” is really more about the processor size. If you need a tiny mainboard then its probably best to avoid Gadgeteer sockets altogether. For a prototyping board, the more sockets the merrier I think.
You know, I get where you’re coming from, but some of us are looking to build small one-offs, not prototypes. I for example would like to stack together a moisture sensor, light module, xbee module, and mainboard into as small as space as I can, without picking up a soldering iron. A cerberus isn’t that big, but the smaller (and less power hungry) the better sometimes.