So, funny story with this. Like many MLP artists, I decided to make Headcanon Heights that are different from the canon show models. Pretty much the only ones unchanged were the Immortals, but on average, Earth Ponies were bigger, Unicorns were smaller, and Pegasai were somewhere inbetween.
 
Now, when I did the Mane 6 Headcanons, I decided to make Twilight enormous for a Unicorn, being the size of an Earth Pony(Not to mention a slight growth spurt from Alicorn Ascension).
 
I hadn’t realized until recently what that meant for certain height comparisons. Flash isn’t actually short. He’s average for a Pegasus. Twi’s just giant. But it makes an adorable comparison , and grumpy Flash is adorable(He and his sisters share the adorable grump face, even though he’s only related to one of them).