Are you eating more carbs than you are burning? That's one way to bulk up.
How to Increase Stomach Acids
Take a supplement for the stomach. It should include Glutamic Acid, Betaine, Pepsin, and sometimes Bromelain. Or just take the amount of HCL Betaine you've determined to be your needed dose per the stomach acid test.
Take a teaspoon or tablespoon of ACV with meals, and eat something vinegared or lacto-fermented such as saeurkraut.
Take 500 mg of unbuffered Vitamin C. (Emergen-C is buffered. Anything that says ascorbate is buffered.)