in my head canon, the ponies preforming the spell is like them thinking, like picture this. a unicorns horn is on the head right? so does that mean that the pony can channel its brainpower into the horn, and like, a special magical gland in the horn turns that brainpower/willpower into raw magic? for instance twilight is a very smart pony, therefore she has a better grip on magic and how much power she has, celestia and luna are smarter than twilight, well most likely, considering that they have been around for so long, you would tend to pick up some stuff. thats my headcanon, for the whole power level situation