I came here to point out exactly this: If you only shrink the ball, without reducing its size, well... you're gonna have problems carrying the ammo.
As a DM, I think I would let them both shrink and reduce the mass, and wait till they fired the weapon before invoking "conservation of momentum" and declaring that the cannon ball drops to the ground after about a meter.
In addition to causing issues for the war machine, causing a fuel shortage also hurts civilians (in a way that is not a war crime). In fact, because the war machine has top priority, a fuel shortage will probably hit civilians first. Frustrated civilians that don't like fuel shortages are more likely to be a nuisance, which is nice.