I noticed how Rarity can attack zombie dresses using magic beams (Do Princesses Dream of Magic Sheep), but when Trixie tries to attack Rainbow Dash with a magic beam it ends up tickling her. (Magic Duel). My theory is that Rarity has more raw power than Trixie, but doesn't know how to utilize it. Trixie on the other hoof probably studied spells, but doesn't have enough magical muscle to pull it off, thus explaining all of her spells under the alicorn amulet. (It only granted the user power, not knowledge)
I noticed how Rarity can attack zombie dresses using magic beams (Do Princesses Dream of Magic Sheep), but when Trixie tries to attack Rainbow Dash with a magic beam it ends up tickling her. (Magic Duel). My theory is that Rarity has more raw power than Trixie, but doesn't know how to utilize it. Trixie on the other hoof probably studied spells, but doesn't have enough magical muscle to pull it off, thus explaining all of her spells under the alicorn amulet. (It only granted the user power, not knowledge)