Why is my battery bulky?

If your phone battery has expanded in size, causing the battery not sit flat, it’s possible that your battery has bloated.

Although this bloating is an inbuilt mechanism to prevent pressure building up inside, they might be dangerous and i would recommend you proceed with caution. (this is not natural state for battery, so it should be changed)

In general, a swollen battery occurs when the battery’s cells are overcharged, as lithium-ion batteries “react unfavorably to overcharging,” according to Don Sadoway, Professor of Materials Chemistry at MIT. As Professor Sadoway explained to Electronics Weekly:

There are strict limits on how much current can be put through a lithium-ion cell. During normal charging, you never see metallic lithium, which is inherently unstable. But during overcharging, the lithium builds up faster than it can dissipate. The result is that metallic lithium plates up on the anode. At the same time, the cathode becomes an oxidizing agent and loses stability.

This reaction produces heat, which warms the gasses inside the battery, causing them to expand. Without any avenue for ventilation, the battery’s casing expands with the gasses, distorting and warping its appearance into that familiar swollen look.