Correct.
A larger cartridge has more inertia and more surface area creating more friction between it and the magazine feed lips and rounds further down in the stack. So, it needs a bigger bolt group to overcome those things, because there is a peak BCG acceleration and recoil spring mass you don't want to exceed, so that means your theoretical BCG velocity is "fixed", and the only way to increase the momentum of the bolt group during feeding is to increase its mass. So no, you could not just drop in 30x173mm rounds even in a hypothetical context, because they'd be too heavy f