The reason that this image has measurements for the height from the ground to the base of there neck (withers) is because this is the method used for measuring the height of ponies and horses in real life, and this length is measured in hands, which is 4 inches. 13.2 would be 13 hands and an additional 2 inches, or 54 inches.

See this link for more information (as well as a table for approximating mass of regular horses): http://horsetalk.co.nz/2013/01/18/working-out-horse-weight/#axzz2n8gd6M6j

This website also provides a way to estimate the mass of a horse based on its dimensions.

Mass in kg = (Girth)^2 * Length / 11000

Where girth and length are measured in centimeters.

8 feet = 243.84cm

Mane 6 Height: 243.84cm * (255/840) * (372/249) = 110.588cm

Mane 6 Girth: 110.588 * (64/265) * pi = 83.906cm

Mane 6 Length: 110.588 * (111/265) = 46.322cm

Mane 6 Mass ~ (girth)^2 * length / 11000 = 29.647 kg

Celestia's Girth: 110.588 * (72/265) * pi = 94.394cm

Celestia's Length: 110.588 * (182/265) = 75.951cm

Celestia's Mass ~ (girth)^2 * length / 11000 = 61.523 kg

It would appear that the ponies here are much less massive than real world ponies.