I expect that the wires are thinner because they are carrying far less current than they once did. Why?
a) LED lamps instead of incandescent lamps.
b) Digital gauges (if any at all) instead of analog gauges.
c) Chip driven diagnostics.

I also expect that Toyota would not move to thinner-gauge wire purely as a matter of first-cost economics if one considers the cost of even a single recall vs. the heavier gauge wire. I also doubt if it is a 'green' decision any more so than any other realized efficiency due to refinements in technology and/or execution.

Peter Wieck
Melrose Park, PA