Here's why:

* Pressure and Volume Relationship: Air is compressible. This means that when you increase the pressure on it, the volume it occupies will decrease.

* Force and Pressure: The force you exert on the plunger translates to pressure on the air inside the syringe. More force means more pressure.

* Syringe as a Closed System: The syringe is essentially a closed system. As you push the plunger in, you can't escape the air, so the pressure has to go somewhere. The only way for the pressure to increase is for the air to compress and occupy a smaller volume.

In short: More force on the plunger = higher pressure inside the syringe = smaller volume of air.