Here is an idea for you.
Premise. A young woman who is on the slightly chubby side (maybe 130/140 pounds) Has just graduated college and is returning home, when she learns that her parents, the only living relatives she had, died in a crash.
At the wake, she meets with a woman who was once her babysitter when she was younger, who was also a friend of her mother. This woman is in her early to mid 40's and she is a large pear-shaped woman (Think bigbootyasshely size).
Having nowhere to go, the daughter moves in with her and they live together. As time goes on, the younger woman starts to put on weight and eventually, she becomes a huge woman who is also pear-shaped (think Maryboberry size). The two woman become close and eventually fall in love with each other. It ends with both woman around the upper 400 to low 500 pound range, with the older woman maybe outweighing the younger woman by about 10-20 pounds.