Boyle's Law states and for a given amount of gas at a constnat temperature, the volume decreases when the pressure increases. This is called an inverse relationship. So, when the amount of pressure decreases, the volume increases.
You can do a test for yourself. If you have a parent or guardian who is a doctor, ask them for a syringe, or just see if you can find one around the house. Place the syringe so there is 15mL of air inside. Put your finger over the front of the syringe. Push in the the back of the syringe, and you'll see the volume decreasing. This is because you are applying pressure to the air inside the syringe, causing the molecules to go closer together, and therefore decrease the volume.