Great question, as the description reads, the data is generated by multiplying the average land elevation of each country by its land area. The result has units of length^3, and is conceptually a measure of how much land is at high altitude.
This is an interesting concept, though it doesn't actually measure how much land is at high elevation. It's just area times average elevation. Since area in km^2 is much higher than even the highest elevation, it skews heavily towards large countries. Case in point, Australia is #6, despite having a high point of only about 7,000 feet (so by most definitions, Australia has little to no land at high elevation), while Afghanistan, with an average elevation of over 6,000 feet, doesn't make the list, despite being the 40th largest country by elevation.
Yes, I chose not to rescale the average elevation, but perhaps I should have to make the quiz more interesting. I agree the list is dominated by large countries and I wanted it to be more specialized.