I am sensitive to good descriptions when I roleplay, often a good description of the fatness on just a chubby body is more effective than just an huge weight gain without description. it’s not only a question of vocabulary but also of context and figure of speech’s use.
I love the weight gain and every details serving as clue witnessing the progression, must be used in all case.
For example if my (in character’s) girlfriend (We will call her Chloe and mine Audrey) started slim she can describe the scene like that :
After open the door of the room and entering, Audrey notice Chloe, relaxing in her armchair, reading a book.
The pocket size book obviously not hide the recent changing on Chloe’s body. Her past flat tummy don’t look so flat anymore and a soft bulge seems to peek a boo under the the tight and rising shirt. Also the small growing roll of flab look touching her fuller thighs.
The distance between her plump hips/thighs and the side of the armchair reduced since the last time.
I can’t give a list of the full small words that can be used in this style of scene, but I advise to read some weight gain story, it’s a fuel of fat vocabulary, scenes and metaphor. 