Bigger ground wires = more copper = lower resistance. The lower resistance mean less voltage drop through the grounds. More voltage to the rest of the system means the alternator doesn't have to produce as much power (because you're not losing as much in the wiring). Less alternator output power means less load on the engine.
For most cases it is better to have fewer grounds. More ground points leads to ground loops and noise susceptibility.
I can see how the gains would be minimal at best, but they would still be there.
