We agree it isn't body length that is the important factor, but disagree over the significance of body mass.
I guess the best way to approach the question is to assume the same jump height and see what comes from that.
The flea weighs 0.4 mg and the human 40 kg, so the human is 100 million times heavier.
To raise the human the same amount as the flea requires 100 million times as much energy.
The human carries around 100 million times as much stored fat, so has 100 million times as much energy available.
Hence both, one jumping up steps and the other walking up them, will use the same proportion of body weight when climbing a stairway of the same height.
To put numbers in, a human would burn off 1% of their body fat climbing 10 km, that is going up the Eiffel Tower 30 times, (it would require a very fit person to have big enough muscles to convert the food energy into height in a reasonable length of time),
If it did scale by mass, a factor of 100 million leaves the flea having to jump 0.1 mm to burn off 1% of it's body mass, and a jump of 10 mm would destroy all of it.
As a proportion of body mass, both the flea and the human burn the same amount by jumping the same height.