There are plenty of synonyms for the term 'become/grow fatter.' Some of the most commonly used words include 'gain weight,' 'put on pounds,' 'expand,' 'enlarge,' 'swell,' 'plump up,' 'bulk up,' 'chub up,' 'pork up,' and 'beef up.' Some more informal terms for growing fatter are 'blimp up,' 'balloon up,' and 'plump out.' Meanwhile, the medical world uses the term 'obesity' to describe extreme weight gain. Regardless of how you describe it, gaining weight is a natural and often necessary part of life, but it's important to make sure this weight gain doesn't lead to other health problems such as heart disease, diabetes, and high blood pressure.