Harry was having a very difficult year. He felt isolated, alone, and angry, and he wasn't entirely sure how to cope. Sometimes, he had visions of harming people. Dreams. Sadistic ones he didn't understand, though didn't entirely dislike. He felt better, being able to fend for himself, for once. No longer the victim, but now the bully--the one with the power and the control.
Even so, these dreams and fantasies hadn't manifested in his real life. He still felt incredibly powerless when he was awake. There were secrets among his friends and his inner circle outside of Hogwarts that he wasn't privy to. He wrote to "Padfoot" regularly, but the replies came slow and were more vague than he would have liked.
He needed to get away for a little while. To clear his head, at least. Being a boy of fifteen, just at the tail end of puberty, wasn't helping him to contain his anger in the least. He found himself venturing the edge of the woods, his wand in hand, always paranoid that he might find himself face to face with He-Who-Must-Not-Be-Named.
Meanwhile, you were out in the woods as well, collecting curious samples of things to bring back to the Ravenclaw dormitory for closer study. Perhaps as evidence of Nargles, at last. They were so crafty in their stealth, it was hard to convince anyone they were real. As you scoured the ground, your bare feet crossed over a twig, and it snapped loudly. Harry couldn't have been more than about ten yards away, and you'd alarmed him. Before you could see what was happening, he threw a spell in panic and self defense.
"Reducio!" he shouted. You saw a bright light engulf your vision. When the light faded, the trees looked huge, and everything around you on the ground seemed huge.
While you were still pondering things through, you saw a gigantic boy come rushing over to you. It was Harry, and he looked like a giant, towering over you, even on his knee. He squinted at you and...