So you, like me, feel that, hypothetically speaking, a pony standing on its hind legs would be about the same height and size as an average human, although slightly shorter (Due to the legs being a bit shorter)?
I just can't picture the ponies being any other size, although that could be because the 'familiar' objects (houses, buildings, plants, etc) are associated in my mind as being a specific size relative to the average human, although I suppose the pony world could be scaled more to the tune of, say, an actual horse.
That being said, units of length could be much different in Equestria than on Earth; perhaps the 'functional equivalent' (Not in terms of exact length, but instead in those of usage) of our 'twelve-inch foot' is similar to the 'cubit' (An ancient form of measure approximately equal to the length of the forearm). Perhaps the 'foreleg' is the base of measure in Equestria, for example. If one takes this into perspective, and assumes that the 'laws of physics' are the same in Equestria as here on Earth, then the size of the average pony, given the strength of muscle, bone and ligaments/tendons, could well exceed that of an average Earth counterpart of a horse. Then again, the opposite is also true; the average 'pony' could be the size of a cat, for all we know.
For simplicity's sake, and the sake of certain subsets of the MLP fanbase (which will go unspecified...), it is easier, and safer, to assume that 'ponies' are similar in size to humans. This being said, the 'cubit', in my case, about 9.5 inches or 24 centimeters, could likely be interchangeable on a 1:1 ratio with a hypothetical unit of measure based on a pony's foreleg, again, assuming ponies have a similar build to a human, possibly notwithstanding the hind legs, which would likely be shorter than our 'legs'.
When one considers the scale of one dimension, or even one world, compared to another, the lines are, sadly, always going to be blurry.