@B P (Ult1m4) By the Grace of the Gods goes into more detail on the problem of
too much mana. Life of a Magic Academy Mage also touches on it. It's a problem of control.
It's like filling a cup. It's easy from a kitchen faucet. A garden hose at full blast is harder. Don't even try with a fire hose.
Ryoma from Grace seems to have a built-in control, and also has had a LOT of practice, so his excessive mana isn't a real problem. But then, his body was explicitly built by the gods when he transferred over.
From Academy, Leehan's country doesn't even allow kids to
try to practice magic until they're legal adults, even Leehan's mighty mages household, and it shows in how hard it is for him to handle his magic at times. It'd probably be worse if he hadn't learned the sword, including aura training. In his Basic Beginning Magic class, the teacher explicitly bans him from practicing with fire due to his difficulty with control. She also compared his magic to the ocean in quantity, once calling it endless like the sea.
The problem comes when he has to use a lot of finely-controlled magic back-to-back in an emergency.
Spoiler!
(Him being in the middle of it was his own fault/choice in the first place.) After less than ten minutes of this, he starts bleeding from the nose and getting headaches. He'd've been in a worse world of hurt if he hadn't been able to switch to the sword to give his brain a break. He winds up using a huge blast of mana at once; he wins, but passes out almost immediately. Which is about when the cavalry arrives, so all's good.
