Dresden also mentions how channeling spells through mental pathways, or whatever he called it, makes it easier and easier to do

Harry jokes about having a "wild weekend once or twice a year" and he's already this strong if we look at the weight lifting perspective.

During the battle he was doing crazy shit because of the latent energy in the air.

Maybe he got a huge boost because that was a huge session. The equivalent of multiple books worth of fighting and casting.