fill out

1.  Complete by supplying required information, especially in writing. For example, Please fill out the application form, or I don't quite understand this drawing, so fill out the details. [Late 1800s]
2.  Become enlarged, distended, rounded in outline. For example, The wind filled out the sails, or He's put on weight and really filled out. Applied to objects, this expression dates from about 1700, but to persons or animals becoming fatter, only from the late 1800s.

The verb has 6 meanings:

Meaning #1: write all the required information onto a form
  Synonyms: complete, fill in, make out

Meaning #2: make bigger or better or more complete
  Synonym: round out

Meaning #3: supplement what is thought to be deficient
  Synonym: eke out

Meaning #4: line or stuff with soft material
  Synonym: pad

Meaning #5: make fat or plump
  Synonyms: fatten, fat, flesh out, plump, plump out, fatten out, fatten up

Meaning #6: become round, plump, or shapely
  Synonym: round