The equestria girl being 1.6 times taller than a normal mare at the ear is consistent with analysis here (adjusting for the high heels) and here (an earlier work of mine, but was build around the same proportions).
 
I’ve calculated from Rarity and Pinkie Pie (whose manes leave the top of their heads unobstructed) that the head height of a mare is 0.95 of their ear height. So a 1.66 (5’5”) meter equestria girl would work out to a mare about 1.04 meters (3’5”) (1.66/1.6) tall at the ears and 0.99 meters (3’3”) (1.040.95) tall at the head.
 
This was posted in response to the latest official source putting mares at about 4’ (1.22 meters). This would put the equestria girls (normal teenage girls) at 1.95 meters (1.221.6), or 6’5”, which is quite improbable. I’ve calculated Principle Celestia (extensive screenshot analysis) is about 1.21 times taller than an equestria girl, so a 1.04 meter pony gets a 1.66 meter girl would gets a 2.01 meter (6’7”) tall Celestia! This is outlandish, but around the height of basketball players, so not too implausible. 3’3” pony height is the smallest consistently calculated size, and anything larger translated into an increasinly implausibly tall Principle Celestia, with the 4’0” pony leading to a whopping 7’9” (2.35 meters) tall Principle Celestia.
 
Explanations (beside the writers/artist failing to fully think things through, but seriously I applaud them for trying to add details like that) include Somnambula being taller than average, (most likely) equestrian feet not being the same as human feet, or (most intestinally) that EG humans are closer to Na’vi than actual humans (this would explain the technicolor skin tones, thin builds, and maybe more). Any other Ideas?