Back to the original subject:
Apologies to the OP for being contradictory, however I would not personally advocate to ground to the chassis - the chassis is not an active current-carrying circuit component on a modern motorcycle, this is not like a car.
There is ZERO current flowing through your frame. Every component's negative return goes through the negative return wiring system directly to the battery negative;
the exception being those engine components - starter, spark plugs, oil pressure and neutral switch- which return via the engine block, then directly to the battery negative via the engine ground cable.
Engine contact to the frame is purely spurious, through painted surfaces; And you will encourage galvanic corrosion at the aluminium - steel interfaces.
So while I am complete agreement with the general premise of increasing the wire size (it has nothing to do with increasing the current capacity, but reducing the resistance) my recommendation would be to just make the cable a bit longer and connect it directly to the engine ground point.
But, similar to the headlight relay upgrades, where to get the maximum benefit, you need to not only augment the positive circuit to the lamps, but also the negative return, you need to do both sides of the starter circuit, which means the battery positive to the solenoid and also the solenoid to the starter.
You need the whole round-trip to maximize the starting efficiency.
This is the result on my Daytona - the difference in cranking speed is significant on that and KNOW the Rocket starter pulls a LOT more current; I guarantee you that it is a lot more than 70A. 4ga might have been overkill on the Daytona - I would probably go with that on the Rocket.
Here's a simple test you can do to determine the upside potential you might have:
Put your bike in gear, leave the side-stand down and tie your clutch lever to the bar with a zip-tie or similar; (that is how I disabled mine from starting in the attached video)
Run your starter (it will crank but won't start) and measure the voltage between the starter lug (on starter itself) to the starter body. Also measure the voltage directly across the battery terminals - this will show how much loss you are getting for the 'round-trip'
If you want to break up the supply and return, measure voltage from battery + to starter terminal and from starter body to negative terminal;
