Actually, a heavier object travelling in a zero-gravity environment will require more force to be applied to change its motion (try stopping a spaceship).
However, this is neither here nor there, as objects in space do not exist in a zero-g environment, even if the only other mass for miles is the enemy...
RL physics cannot be applied to FL.
And, back on topic, adding a downfall to armor upgrades will wipe them out completely, as most people will just use the unarmored versions because they are 15 mill (75%) cheaper.