fill out vs lengthen

fill out

verb
  • To have one's physique expand with maturity or with surplus weight. 

  • To complete a form or questionnaire with requested information. 

lengthen

verb
  • To make longer, to extend the length of. 

  • To become longer. 

How often have the words fill out and lengthen occurred in a corpus of books? (source: Google Ngram Viewer )