It's quite interesting to note that some of the most powerful and old magic actually are not fueled by Magic, but by other things (the amount of magic is likely only used to start the process of drawing that something else):
- Last Aegis uses Remaining Time,
- Fairy Glitter uses gathered and concentrated Light,
- Fairy Sphere uses Bonds and Faith,
- probably more (like using fear and the like).
I guess Lucy's comment about the Origin of Magic being the Heart doesn't seem so far away from the truth.