Genisay

I had to laugh. So far, I think the best theory I have heard/seen came from my Roomie, who doesn't even watch the show, but hears me and some of her other friends talk about it. Her theory is that it is related to Magical Power. Princess Celestia has been established as currently being the strongest 'Good' power in the series. She may very well be a marker for the tallest they might get. Princess Luna and Cadence are younger, possibly by quite a bit, than Celestia, and their powers are not yet a strong as hers, but still far stronger then that of the average Unicorn, Sky Pony, or Earth Pony, who all seem to represent the standard height, as well as magical ability. The theory was backed up by Luna being shorter when her power was diminished after being freed from the Night Mare Moon form, and then she gains height again as she returns to her full power. (Borrowing from Lauren Faust's own unofficial quote.) Cadence is also closer to the average pony height when she is still Twilight's foal sitter, but is closer to Luna's height when they meet again. I think this Theory would also support that as her magic grows through training and refinement, Twilight Sparkle could also eventually grow taller. This might also explain why Shining Armor is also tall, (beyond being a stallion as well, so having a bit of extra height already, as the Stallions in general seem to be taller then their female counterparts, even if by just a little), as well as King Somber, and Queen Chrysalis. All of these ponies have been proven to have far stronger then normal magical powers, and are also taller then the rest. (Except Twili, not yet at least). Don't know if this is it for sure, but it did cover a lot of bases, and was a pretty solid theory as to how that could all work.