There wasn't much of a chance or a choice for Diana to hold onto that power or let it go. Thor knew fully what a dangerous foe Hela was by herself, and adding the Enchantress too? Even more so. He hadn't held back, not even a little bit. There were no people around that he could see, no animals, not even birds in the sky. So why bother holding back? Why not put her down (again) before she could do any more major harm? What happened to Asgard could happen here if he didn't.
So when Diana took that hit, she was taking the equivalent of enough energy to power the entire city for as much as a few days. It was not a small amount of energy, and when Diana radiated it back out, only even more so. She'd amplified it just slightly, maybe a modulation of between one and three percent, but that was an order of magnitudes compared to a single lightning strike.
Glass burst in the windows of every nearby building, grass burned and charred in an instant, and anything not Diana took an explosive, heavy hit of energy, including Thor himself. When Diana came down from her momentary burst of power, she found Thor bashed into another building, unconscious for the moment. Hela and Amora were gone, with six charred forms etched into the ground between Thor and Diana. He wouldn't stay out for long, and was already stirring by the time Diana noticed what happened.