www.bfwpub.com/pdfs/kolbwhishaw5e/ch02part2.pdf

See Figure 2.6. There is a diagonal line (i.e. n=1) on the plot.

It says: "Modern humans have the largest brain relative to body size of any animal and lie farthest away from the diagonal line."

Here the "any animal" includes Vampire Bat. Maybe the two figures are not exactly same.