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It's definitely more about weight than saving power. Usually there's several pounds of excess steel in all the pulleys that doesn't really serve much of a purpose; replacing with aluminum or FRP is an easy way to take a bunch of mass off the nose of the engine.
AFAIK aside from low efficiency alternators, the belt-driven water pump is the biggest parasitic load since they have no pressure relief valve and cavitate at high rpm. Unfortunately, belt routing can make it a challenge to electrify the water pump. If all your accessories are belt driven instead of electric, a slightly smaller diameter crank pulley is a clean way to reduce overall wasted power.
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