This graph was constructed with tongue in cheek. Though the data are real, this is hardly a serious matter, and is presented primarily for amusement.
One might expect that, all else being equal, a larger map would command a higher price than a smaller map. The following graph suggests that this is the case. This graph plots all maps from the database (atlases, globes, and other non-map items are excluded) - 83,823 items in all - showing the area (in cm2) against price. Just to add a twist to an admittedly strange graph, auction results (blue) are differentiated from dealer catalogue listings (red).
Certainly there is an overall lower-left to upper-right trend, and this is verified by the angled lines, which show the best straight-line estimate (in technical terms: least-squares linear fit) of the relationship between size and price, given the actual data points. But the wide spread of points well away from the fitted lines indicates that there are other factors which, taken together, are much more related to price than size. In fact, from a mathematical point of view, the correlation between size and price indicates that only 15% of the variation in prices can be attributed to size; the other 85% is due to other factors. (As you can see from the nearly parallel lines, there isn't any notable difference between auction entries and dealer entries, other than that the auction entry fit indicates a higher price, on average, for a map of a given size, than the corresponding dealer fit.)
Both size and price scales are plotted on logarithmic axes, due to the wide variation and skewed distribution of the values. To get a better intuitive sense of the size dimension, two dashed lines are shown at the positions that correspond roughly to the size of a typical map from an edition of the Atlas Minor and to the size of a typical map from a folio atlas.
In case you are curious, the smallest map included has an area of 18 cm2 (a Lamberti playing-card map), while the largest has an area of 103,600 cm2 (a 25-sheet map of Belgium by Ferraris, unassembled; the size given is if assembled).
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