You are confusing the size of the eyeball and the gap between the eyelids we see through. The size of the Eyeball is being inferred from the eye sockets in the skulls.
The logic would be, that as there is less light available near the poles, people have evolved larger eyes to capture more of what is available.
However, there are some points of concern with this study.
Why such a small sample?
With such a small sample there must be a large probability of error (without the original numbers this cannot be calculated).
The choice to plot against the midpoint of the country of origin seems strange (why not a population weighted centre of gravity - or any number of possible options including analysis of elements in the teeth to establish where they grew up).
There are also concerns regarding who the "indigenous" people of each country / area are. Take England (and by England, do they mean the UK?) - if so, the country is almost 1000 miles from N to S and contains a number of "indigenous" peoples, all of which arrived from somewhere else in the more or less recent past)