The Anandtech breakdown (https://www.anandtech.com/print/18871/arm-unveils-armv92-mob...) goes into detail on this. While all the cores are claimed to perform better at the same power draw, they draw less power if they have to perform at the same level as the previous generation.

Smartphones will use a combination of big (X-4), medium (A720) and small (A520). Since the small and medium cores are more performant and use less power, that opens up opportunities for battery savings. There could be loads where the larger core remains asleep while the smaller ones finish the job, since they’re more capable. Or the same cores do the job, but use less power.

So if you buy a flagship in a couple of years, the CPU will consume less power. The battery tech will have progressed as well. However, the screens might be higher res or higher refresh rate (120Hz instead of 60Hz). Or it might use a 5G radio that needs more power. So the net effect is that battery lasts about a day.