Pumping up bicycle tires, why does the mass increase and the density increase?

Because the volume will not change much, if you pump it in, the mass will increase, the pressure will increase, the density will increase and the molecular spacing will decrease.

Air is compressible, just like a medical syringe. You block your head and press it in, and the air is compressed. This is the same mass, smaller and denser.

Is it easy to understand?