fatten up

verb

1. To cause to gain weight by means of feeding.

   “My cat looks underweight, so I'll fatten him up with higher-calorie cat food.”

   “I visited my mother for a month, and she managed to fatten me up 20 pounds.”

2. To become fat or fatter by gaining weight.

   “‘And isn’t Declan looking very thin?’ Madge said. ‘He’ll never get a wife if he doesn’t fatten up a bit.’”

3. To make bigger; to expand.

   “We must not fatten up our payroll with people performing nonessential activities.”