Regular meals and exercise will help fill out that scrawny physique in a few months.
After a few weeks of eating well, the malnourished child began to fill out.
During the holiday season, the formerly skinny turkey is plump enough to fill out the entire oven.
This winter, regular consumption of nutritious meals will fill out the underweight athlete.
By the summer holidays, they expect the child who suffered from an eating disorder to fill out and be healthy again.

Supplement what is thought to be deficient; "He eked out his meager pay by giving private lessons"; "Braque eked out his collages with charcoal".

Verbs of buying, selling, owning.

She tried to fill out the questionnaire as quickly as possible but found some sections required extensive personal details.
He fill out the spaces in the room with an array of eclectic furniture pieces.
The writer managed to fully engage the audience by first trying to fill out character descriptions for the story’s main cast.
Filling out his passport proved to be an ordeal due to its various personal and physical documentation demands.
Every Monday, office employees fill out time sheets detailing the total hours they had worked the previous week.