Proportions are a really difficult thing to work with - even in real life, they don’t always make sense.
 
For instance, the midpoint of a human’s height according to standard proportions is the point where the legs meet (ie. the genitals). So the length of the leg should be = length of upperbody + head.
 
My legs = 30ins long. If that proportion held true, I’d be 60ins = 5ft tall.  
My upperbody + head = 40ins long. If the proportions held true, I’d be 80 ins = 6ft 8ins tall!  
Actually I’m exactly halfway between those, at 70ins = 5ft 10ins. But with really short legs. My sister likes to call them “stumpy”. :P
 
Tl;dr - you can’t argue stuff based on generalised proportions. Also why assume that animals are the same size in both worlds? Maybe bunnies in Equestria are really tiny? :)

remember, the humans in eqg are wearing some serious pumps
 
they’re not so freakishly tall and their legs aren’t so freakishly long when you figure out that their boots are elevators. cut an inch off at least.

@Nosfrat  
And Sunset Shimmer as demon has about 8 feet. Damn, Equestrian girls are really tall.

@keanuXdante  
…which would make said (FEMALE) adults stand at well over 2 meters (6.6). That, or Fluttershy is hardly 4 feet.

celestia and luna were adults in the movie, and they stood about 1/2 - 3/4 of a head higher than the girls. flutters is slouching in this picture, so i think its a safe bet that the “average” adult in equestria girls is the same height as the tip of iron will’s horns.

That’s some Bag of Holding Flutters has!  
;)

This makes the measure-everything-with-apples look ridiculous. All that we need, now, is Comparing Fluttershy to Principal Celestia.

I can tell by the size of the plot that that is season 1 celestia

@Background Pony  
Still, you wouldn’t have to look down nearly as much at her as you would any other pony.
 
Guess Iron Will is a better approximation.

@Itsthinking
 
I know.  
According to this, she isn’t eye level with an “average human”, she’s eye level with the average highschool girl.

@Background Pony  
I think he’s talking about Celestia. Celestia is eye-level with the average human.

@PonyPon
 
Remember, the mane 6 are teenagers in the movie.

She’s about level with the average human.  
I could look her in the eye without moving my head.