This problem probably occurs due to the fact I don't see you deleting the old fixture (physics_fixture_delete) before binding the new one. You MUST do this or else you get a memory leak and will run into problems (like this one) due to overlapping, multiple fixtures on the same instance.
Thank you for reminding me! I did this but the problem still persists
In fact, you should delete a fixture as soon as you're done binding it to instances.
As for your problem, I suspect that you're binding a new fixture to the instance but not unbinding the old ones. Unfortunately, I see no function to unbind a fixture from an instance.
Could someone confirm my guess?
Ah, okay. I just made it so that it destroys itself and creates a new objGuy whenever you press up or down (and adjusts the size). Not ideal, but it made it work.