Blood Knights are supposed to look larger because they're more deadly than all other cavalry in the game, and in the background. They're supposed to look intimidating and dangerous, and giving them a larger profile than other cavalry models achieves that look, while making them look less intimidating than monstrous cavalry units. But you should know that already.
Anyway thanks for the photos Borgnine. The Mournful Knights aren't as small as I thought they would be but they are still significantly smaller than the Blood Knights.