One of the explanations I had for how Jensen can travel those air vents so well is he had detachable limbs and he would just slitther inside like a snake.

However, he can still shoot while doing it so I ran out of ideas. Some of the guards even point you would need to be a midget to travel those tunnels.

Turning into a duck is as possible as breaking the laws of physics to fit there. I personally prefer the duck option.