Victor Hugo was born to make Tolstoy seem laconic. Tolstoy can be prolix, but Austerlitz and Borodino are critical to the action of the novel. Waterloo has a minor plot justification inn Les Miserable, but most of the twelve chapters on Waterloo are just Hugo's excuse for spinning his theories.
I'm pretty sure Hugo being a fan of Napoleon helped.
Quite. But somebody commenting mentioned Stendhal, who greatly admired Napoleon, and who managed to keep his account of Waterloo far shorter.
Good point. I don't know very well Stendhal, thank you.