On the K3, the smallest connector is the ground pin in the bottom left slot, which is also used for serial communications. It is a very small pin so limits electrical current, but you *could* also use the USB jack for a better ground. The power pin in the top slot is quite large.
On the K4 and K5, the square pads on the back are ferrous so that you can connect to them with magnetic power connectors. To connect to these power pads, you could use tiny magnets taken from "magnetic stick and ball toys" (e.g. Magnetix brand), with wires attached, but then you would violate a patent for the obvious use of using magnets to attach a power connector (owned by Apple).
On the K4 and K5, the ground pad is just above the USB jack, and power to the other jack is controlled by software (you can turn in on and off in diags). The K4 and K5 power pads should be able to handle more current than the K3 ground pin.