From a game play perspective, G40 and G41 help enforce the GDC's envisioned game mechanics. Boulders are supposed to be carried, one by one, into the courtyard where they are used to weaken the tower. No tossing them over or bowling them through the outer works. No shortcuts through the other alliance's secret passage. I don't think it's an accident that a breach requires the same number of crossings as weakening the tower requires boulders.
From a thematic/historical perspective, all the same and even more so. Geologists classify sediments according to grain size, most commonly using the "
phi scale". On the phi scale, boulders are (rather curiously) specified as stones having a minimum diameter of 10.1", and typically massing at least a hundredweight. Boulders are not to be "tossed", or otherwise casually propelled. Boulders should be carried deliberately to where our siege engine launches them to weaken the enemy's defenses.
Edit: Well played on both fronts, GDC.