People like to get in hot fights about this all the time, and I could be taking the bait, but there is an overhead to instruction decoding and fetching that changes with the complexity of the encoding, and there is also a cost of implementing all the instructions.

Simpler instruction set means fewer gates means less power draw, with some hand waving. The ARM 1 had 25,000 transistors and the ARM 2 had 30,000. The contemporaneous Intel 386 had 280,000 and the 486 had 1,200,000. Intel had the engineering resources to design powerful chips that people bought in droves, ARM had a very small number of engineers and had to design something smaller but as a consequence the ended up with very low power consumption, which wasn’t important until people put it in phones. Since then Intel has optimized for power consumption but they were catching up for a long time.

In most modern CPUs, other factors dominate and instruction set is less important. Nowadays we have plenty of gates to spare and the question is how much can you accomplish at a given price and power envelope. And it’s irrelevant to talk about power consumption of a baseband processor unless you’re also talking about the power consumption of the radio, since they get used together.