From Melvin Konner’s The Evolution of Childhood:

In horse evolution, the face lengthened faster than the body size increased because face length is determined by the area of tooth surface needed for chewing, which, since it tracks the amount eaten, depends on body mass.  Body mass increases roughly as the cube of body length, and tooth surface as the square of face length, so face length must increase faster than body length, a prediction confirmed by the fossil record and interspecies comparisons.

The whole book’s full of interesting empirical work I was barely aware existed, especially trait regressions where each species is a data point.