I misunderstood the methodology as well (thinking the length of the border was important) and asked myself the same question. But now that I get it Chile is definitely out of the picture. I mean it's not thaaaaat big of a country even though its elongated shape makes you think otherwise.
Russia is rank 1 in the world by land area and China is rank 3. So the average of the two is 2.
Likewise Canada=2 and USA=4; Average=3
Got Angola with 1 second left
https://en.wikipedia.org/wiki/List_of_countries_and_dependencies_by_area#cite_note-3rd-largest-14
But why isn't Chile - Argentina on the list?