Smaller transistors mean fewer electrons that go in and out every clock cycle, so less power per transistor.

Higher clock frequency means more cycles per second, that is more electrons spent per second thus higher power consumption.

Since clock frequency has stabilised and the total area of a chip is I think not much larger than before, it is expected to see the power consumption stabilising.

I also believe I read somewhere that one of the reasons clock frequency stoped increasing was that the power consumption became too high for the chips to handle the thermal dissipation.