On a freeway, unsprung weight has little to no effect on efficiency. The 2 main things are aerodynamic drag and rolling resistance. The wheels (ie hubs) contribute to drag. In the example above, the Tesla aero wheels probably had more effect than anything else. The force required to punch through the air is proportional to velocity squared. A tiny bit more aerodynamic efficiency can have a big effect.
Rolling resistance is how much energy it takes to roll a wheel. A wheel (tyre) which deforms, uses energy to deform. A solid train wheel doesn't deform and has near zero rolling resistance. A sticky, wide tyre needs a lot of energy to deform, so it has a high rolling resistance. With everything else equal, a thinner tyre should need less energy to deform. Similarly, a tyre pumped up to a higher inflation deforms less, so has less rolling resistance. A smaller tyre has less rubber to deform, so has lower rolling resistance. Eco tyres have low rolling resistance, which means the energy required to deform them is lower. This lowers the rolling resistance, hence improves efficiency.
So, are smaller wheels more efficient? Not necessarily. For the same overall dimensions, you have more rubber to deform. Width, compound and tread play a bigger part. Lowering unsprung weight helps handling and performance, but probably doesn't improve efficiency.
Another example. I replaced my eagle f1s with ad-09.. they are the same physical size, but the ad-09 are softer and have more rubber touching the ground (more aggressive tread pattern). They are 10-15% less efficient, but oh so much more fun
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