ARM chips are heterogenous, so while they may claim to have 8 cores, not all of those cores are the same. Some are low-power cores meant to save battery power, and the high power cores are only activated when necessary.

Plus regardless of that, just because a CPU is old doesn't mean it's more power efficient than a new one. If anything, it's usually the other way around.

edit: more info at https://en.wikipedia.org/wiki/ARM_big.LITTLE