Following up on the question "What curse did Peter Pettigrew use to kill thirteen muggles at once?", how come large-scale spells are quite rare / not used often in the Harry Potter world?
Off the top of my head I can recall:
- the fire snake scene in the room of requirement
- Dumbledore fighting off the inferi at the Horcrux-lake
- the defense shield in the Battle of Hogwarts
Pardon for not remembering the details, surely someone can chip in to correct me if I called something wrong.
Why are large-scale spells not used often, especially in many-on-many battles?
One possible explanation is that they're simply too difficult for a single wizard to manage alone. The defense shield over Hogwarts might be a sample of this, as, if I recall correctly off the top of my head, it was a lot of small spells, not one large spell.
However from the first reference in the above list, we might see a counter-example, as Goyle isn't generally shown to be the mightiest of wizards, yet he creates a large-scale inferno.
We can see that Dumbledore, possibly the greatest wizard of his era, can manage such spells even after being severely hurt (see the reference in the list above), wouldn't other wizards of similar scale, for example, Voldemort, be able to use such spells to fight off a number of enemies at a time easily?
If there are any canon explanations that would be great, but logical reasoning will do as well.