Celestia: Shh, here comes Discord my dear sister. Luna: This is going to be a good scare. Discord: Ahh, what a beautiful day for someone like me. Celestia and Nightmare Moon: BOO! Discord: (screams of fear.) Celestia: Oh man Discord, you should've seen the look on your face. Luna: FOR THE TROLLING HAS BEEN DOUBLED!
It's Ok. I understand It just makes more sense if they were around the size of a horse considering they are much larger than the ponies. (At least Celestia would be, Luna would probably be in between.)