After scrolling for a while, Sarah saw a fetish called "weight gain", by far the most interesting of all of them for her. She applied it as a fetish for Jeff. When confirmed the changes, she enabled that Jeff's memories would be modified.
Jeff at the toilet noticed his flat stomach, how he always wanted it to be bigger, himself to be fatter. He always wanted to gain weight but could never. With the chronivac he could change that.
Coming back, Jeff asked Sarah if he could get heavier, she could only accept his request. He made himself overweight, he did not want to change too much and become too heavy. He also made the chronivac change reality, so he always had a tendency to be too heavy.
When the transformation was complete, he noticed how he had changed, he was slobbier. He also noticed how Sarah changed, being more motivated to cook and feed him, something he always accepted and appreciated.