Increasing only part of a wire gauge reduces resistance.
It does in a water pipe too, but not as much as with electricity - that's one instance where the water model/analogy isn't that good.
But when talking about grounding kits as in the big 3 etc you replace the engine/chassis/battery grounds with fatter wires, or add extra wires. (Adding extra independent cables/wires/straps is better because (1) it does not impact warranties & (2) provides redundancy.)
In general those big-3 grounds are the most or VERY important because a bad ground can blow electronics. VIZ some vehicle that blew $thousands of equipment (7 DVD screens etc) which I suspect was due to a poor or broken engine to chassis/body GND or similar.
FYI - as I recently wrote on the12volt.com, vehicle GNDs are usually battery- to engine (mainly for the startermotor), and battery- to chassis/body (that's TWO grounds to inter-ground those 3 vehicle components), however the latter could be engine to chassis/body instead, or that engine-chassis/body could be added to provide redundancy or reduce overall ground resistance.
Incidentally that shows one ambiguity - I regarded the BIG-3 as including the +12V (eg, alternator to battery or battery to +12V system etc) but apparently it meant having 3 grounds (battery- engine chassis/body).
You are not wrong in your thinking, but if talking big-3 type grounding kits, they are to ensure SOLID grounding between engine & battery and "the vehicle system" (body etc).
Equipment grounding IMO is essentially a separate issue. F.ex even with solid ground bonding you might still take power & GND straight from the battery. And then wanting a "common ground point" (to avoid ground loops or differentials) might mean other GNDs are also from the battery.
Usually a chassis point is chosen for a common & compromise GND point since (1) it's difficult mounting a lot of GNDs on the batt- terminal; (2) engine or alternator grounding means vibration & heat (increased resistance & breakdown) and similar crowding or mounting difficulties, & (3) it provides a compromise mid-point ground (for those that talk about the battery or alternator being the "absolute ground").
I discussed the above on the12volt's 2011 jeep liberty big three wire upgrade (page 2).