If each side is 0.1 feet extra,
The volume will be 5.1*2.1*1.1= about 11.781.
Perhaps this helps.
The motivation behind why the vertical stature of the stairs is the main thing measured is that it uncovers to us how much gravity is up against the individual and their weight, so we require this data to decide how much vitality and power we have to get up the stairs.
