[vanderZwan]

The lower on the graph, the more energy efficient. The further right on the graph, the heavier the object.

So the trend is towards more efficiency per kilogram for heavier animals, presumably due to square-cube law[0]. Or maybe the 3/4 scaling law[1][2].

[0] https://en.wikipedia.org/wiki/Square%E2%80%93cube_law

[1] http://mathbench.umd.edu/modules/misc_scaling/page20.htm

[2] https://www.youtube.com/watch?v=DFFVSvAr7Wc