Why did your battery get fat?

If you mean the battery is bulging or swelling then it may have been frozen. This will cause the electrolyte to freeze and bulge. A battery that is being or has been overcharged will also bulge or swell. A battery that is not secure and bounces around can have internal damage to the plates will can cause them to explode and bulge the battery. In any case I would immediately have your charging system tested at an auto parts store as your voltage regulator may be putting too much voltage into the battery. You should get a reading of 13.5 to 15.5 volts DC with the engine running at idle when you test at the battery posts. No matter what you find, replace this battery as it is dangerous and can explode at any time.