You'd have to know how magic works to answer.
If the magic bears any resemblance to known physics, the energy has to come from somewhere. Does it come from the chemical energy in the magician's body? That would make throwing fireballs that destroy a building impossible. Maybe he uses the chemical energy in his body like some sort of catalyst to move energy in the surrounding world, like pushing a boulder over a cliff. Maybe the energy comes from a pool of mana that's just out there waiting for a magician to draw on it. Maybe there's some mechanism to manipulate mana, akin to burning oil to make an engine run.
If you want your story to have magic that operates according to rational rules, you'll have to figure out what those rules are. Lots of fantasy stories don't worry about it, they just declare that the magician can throw fire balls or raise zombie armies or whatever, and that's it.
Personally, I always balk at stories where the magic is not governed by any discernable rules -- other than, "what suits to the author to advance the plot at this point". I really dislike stories where in chapter 1 the wizard can throw a fireball that destroys a city but in chapter 5 he is overpowered and captured by an ordinary guy with a club, with no explanation why he couldn't destroy his attacker with a similar fireball. But maybe that's just me.
If I was writing a fantasy story, I'd try to work out some "theory" about how magic works. Like let's say there's this form of energy unknown to 21st century physicists. Let's call it "mana". And let's say it can be manipulated by saying just the right words and making just the right hand gestures. Then it follows that the way for a magician to increase his power is to study the workings of mana and learn more about what effect any given hand gesture, etc, has. He could do research in much the way that a chemist or physicist does research. Once he knows the principles, he probably needs to practice and train to get the hand gestures just right.
Another obvious theory would be that there are good and evil spirits in the world, ghosts or whatever, and that magic works by convincing or forcing these beings to do your will.
I'm sure many other theories are possible, in the sense of, would be internally consistent and at least vaguely plausible.