Similar to overall performance, power consumption has many factors. As others have pointed out, a new process is a big part of it. Additionally, they have very efficient and well designed small cores in their SoC and one benefit their full vertical integration is their OS has very good schedulers for maximizing time on small power-efficient cores without impacting performance in a noticeable way. At a core level, I assume they have very good clock and power gating, so even if the cores have these massive structures, they might be able to power down some or all of them. For instance, adding a 4th vector unit might seem expensive, but most workloads are not running vector instructions, so all 4 of those units can be powered off for the time slice that application is running. Apple also has very good physical design teams that produce custom macros for almost the entire chip, which individually are very minor but do add up significantly.

I suspect that's largely due to the smaller process size.