Oh wow, sorry–I completely forgot about this! ^^; Thank you for reminding me!
That said… have you updated your code? I downloaded it today, and I don’t seem to be encountering the same issue that you demonstrated above. Nor do I see the cube-floors that you seem to have there. (And I note that the character seems to have been replaced, suggesting too that this may be a newer version.)
One thing that I note, by the way, is that jumping seems to be strongly affected by the game’s frame-rate: your game runs very fast indeed on my machine, and as a result, jumping barely functions. Conversely, if I limit the frame-rate then I can suddenly jump very high indeed.
I would imagine that this is due to jumping currently being implemented as a force. Forces are applied over time–thus the longer they persist, the more effect they have.
In this case, you clear the force in the update function, and thus I imagine that this force only persists for one frame. As a result, the duration over which the force is applied is the duration of a single frame. Thus, the longer the time between frames (i.e. the lower the frame-rate), the longer the time over which the force is applied, and the higher the jump.
Presuming that this isn’t what you want to have happen, what you might want is an instantaneous impulse–essentially (as used here) a single, instant, unvarying change in velocity, regardless of any duration.
I think that this can be done by calling “add_impulse” on your PhysicsObject, instead of applying a force.