Until the series finale I had always assumed that the princesses' sizes didn't actually have much to do with them being alicorns. We've seen other ponies with similar body types. Maybe the flowing mane only comes after a few centuries, or maybe it's specific to Celestia and Luna for some reason.
It was interesting that Cadance didn't have a flowing mane. I just thought she wasn't old enough even though she has been an alicorn for more than a while, so I also wonder how much time passed for "The Last Problem".