You feel lighter because you feel more balanced! Posture is how you balance your body. When you are adjusted, joints which have been unable to move in harmony with the rest of the body are mobilized. So you can balance more effectively (bio-mechanically speaking).

Try this: Take a broom, and balance it on end, straight up in the air keeping only one hand on the handle. If you keep it straight, it is easy to balance. If you let the broom lean over to one side, it gets A LOT heavier. Your posture is the same way. If you keep it balanced, it takes less energy to hold yourself up.