Nexus Q Teardown
Well, the capacitor is rated at 400V, but I'm very confident it's not actually having 400V applied to it in this case. It's almost certainly a power supply filtering capacitor, so it's just taking 120-240V wall current.
Nexus 7 Teardown
And that extra millimeter is partly due to the smaller height and width--a larger tablet could be even thinner because the battery could be spread out.