What's up with these configurations

Wow first of all you all really took off on my question "To plug into or not in a GFCI outlet" thanks to all who put their knowledge and experience into answering that!

These configurations in wireing I have seen presently while working in the historical district where these houses used to be rentals and now they are being gutted out and totally reconditioned.

An outlet box in the living room (house mid to late 50's built) that has 10AWG for a standard 15A duplex receptacle. Wouldn't 12AWG be plenty enough? Why 10AWG?

Apparently in another outlet box with 4 10AWG wires 2 hot,2 neutral in the bedroom of same house a GFI outlet was wired in wire nutting two of the 10AWG'S hots together ,and also under that same wire nut was a pigtail 12AWG.
The other end of that 12AWG was connected to the brass screw terminal on the LINE side,and the same was done for the neutral side connecting to the silver screw terminal to the other LINE side. I was not aware that AWG's could be mixed like that? Or can they?

Thanks all!

The 10GA may be derated either for length or bundling. Its just a code requirement to up the gauge for certain circumstances. Or its what the "electrician" had extra or left over as in the case of the outdoor romex all over my basement.

The GFCI is fine as long as the supply is no more than 20 amps. The pigtails make it easier to connect to the GFCI.

Awg

The smallest wire size in the circuit determines the circuit breaker size, as long as you do not create a situation where someone might misinterpret the circuit and increase the circuit breaker capcity.

After 50 or 60 years, you will find all kinds of damage done by handymen and homeowners.

The only stock #10 wire I have seen on general lighting/recep circuits was in the early 70's stuff where they uses AL or copper clad AL wiring. Everything is upsized one size.

As far as work done after it was built, nothing surprises me anymore.