Back to my shotgun analogy: different weight loads recoil different amounts. This has nothing to do with pressure, as they all push on the same amount of air with the same amount of force. It has everything to do with the mass of the projectile(s) being propelled out of the barrel.
Again, this is direct evidence of Newton's 3rd law, which is what rockets operate on, and why they are more effective in a vacuum, not less. And before you say "yes, but rockets don't fire shot", realise that all matter (including gas) has mass, and the mass is what matters. (No pun intended!)
Oh, and going to a university and doing the balloon in a vacuum chamber experiment suggested by icanbeanything would be well worth your time. Just don't blow it up too full to start with.