arm in a lot of chip what makes the power efficient is not the architecture but the micro-architecture that why you can see x86 cpu having a better power concemption than some arm chip.
but arm is more designed for low power this is manly due to :
- simple, fixed-length instructions (32)
- no 1-15 byte length
- cheaper decoder (1–2 mm² vs 6–10 mm² on x86)
- sve better power scaling than avx