Because less transistors need to switch.

I'm honestly curious how that plays out in practice. 64b Intel CPUs can run in 32b mode. If we count actual switching then we would end up with ~same usage. But in 64b mode it gives us both longer pointers and offsets it with fewer instructions on vector operations.

I could not find any actual paper with real use case comparison. Have you got one?

I haven't got a reference, and would be interested in one too.

Sure, if you use any kind of 32bit mode then you'd use less energy in theory. But if we leave the responsibility to the software developer, then we all know what happens :)

How much energy would this save? Of all the energy usage on the planet, this seems like it would be really really tiny.