72 duster firewall wiring questions ???

I did find it interesting that the blue wire under the hood is a 16 or smaller gauge wire but on the interior side of the bulkhead it's a 10-12 gauge...? Seemed strange to me?? I put a 14 gauge lead on it at the 3 way splice and was planning on running it all the way back to the ignition switch, but may keep the factory heavier gauge wire if the landed end is good @ the switch.