Think of it in terms of the size of each stair and the size of the person.

If each stair is > the size of my leg, each step is plenty of work. Doubling it engages more muscle groups. In this case obviously more energy.

If each stair is < the sole of my shoe, each step is barely any work. Doubling it is negligible, no new muscles are engaged, they just work slightly harder for double the gain.

So, the answer is both, and it depends on the size of the person and the size of the stairs.