big cat vs tigress

big cat

noun
  • Any large feline animal, especially one of the Panthera genus, including tigers, lions, jaguars and leopards. 

  • Used other than figuratively or idiomatically: see big, cat. 

tigress

noun
  • A female tiger; a she-tiger. 

How often have the words big cat and tigress occurred in a corpus of books? (source: Google Ngram Viewer )