This is just basic outlet wiring practices and how to follow electrical codes.
A 15-amp circuit should be wired with 14 ga. only. Using larger ga. wire is a fools errand. In this situation, you can basically overload the circuit and not trip your protective circuit breaker in your service panel.
A 20-amp circuit should be wired with 12 ga. only. Same thing applies as above.
Whether or not an outlet can physically be wired with a larger conductor/gauge wire is unsafe and is an electrical code violation.
Using multiple outlets to the same room location on different circuits is an invitation for ground loops. In an attempt to avoid ground loops, you should locate those circuit breakers on different bus locations in your service panel prior to finalizing their locations. You may need to re-arrange them, so leave the service panel cover off until you ensure that you don't have a ground loop. If/when you are successful, then you can break-off the service panel breaker covers at their finalized locations.
Just because someone else wires their breakers/outlets with oversized wire doesn't make it right.