The wires in your Toyota may not be thinner. Every manufacturer uses the same formulas and guidelines for determining wire gauge based on load. Some manufacturers will have a minimum size they will use, based mostly on the types of connectors that they want to use/have qualified for their vehicles.

Your Toyota wiring may have thinner insulation. Recent advances in insulation materials have allowed for smaller diameter / thickness insulation that saves weight and bulk, and therefore fuel and possibly cost, although some of the newer insulation types are significantly more expensive than the older materials.

So in fact, it's possivble that your wire harness is of better quality and more expensive than some other makes....

There is so much cross pollination between manufacturers now that the differences between foreign & domestic, or even just maker1 to maker2.... are essentially meaningless.