Li-Ion Battery Swelling Mechanism
Most of the time the swilling is due to a physical distruction of the separator between the two battery poles which leads to a chemical reaction between the two electrolytes. 

Although you can still measure battery voltage, I assume that no significant current is available. 

I strongly recommend storing the battery in a fireprove environment and bring it to a recycling facility as soon as possible. Under no circumstances I would try to recharge the device.