prey on vs tip the scales at

prey on

verb
  • To weigh heavily upon (a person's mind). 

  • To eat (as prey). 

  • To exploit or victimize. 

tip the scales at

verb
  • To weigh in at; to weigh. 

How often have the words prey on and tip the scales at occurred in a corpus of books? (source: Google Ngram Viewer )