fatten up

verb3

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.’ — 1999
3. •
   (figurative) To make bigger; to expand.

   • We must not fatten up our payroll with people performing nonessential activities. — 1954