bulk out

1. Of a person, to become larger and more muscular. The coach said I needed to bulk out if I wanted a chance of playing on the varsity team next season. I prefer to focus on endurance and muscle tone when I go to the gym, not on bulking out.

2. To cause something to become larger in size, scope, or substance by or as by adding additional content or material to it. In this usage, a noun or pronoun can be used between "bulk" and "out." I thought I was finished with my novel, but my editor wants me to bulk it out with more world building. We should really bulk out our inventory a bit ahead of the holiday season this year. Adding chickpeas is a great way to bulk out the stew.