decrease vs weaken

decrease

verb
  • To make (a quantity) smaller. 

  • Of a quantity, to become smaller. 

noun
  • A reduction in the number of stitches, usually accomplished by suspending the stitch to be decreased from another existing stitch or by knitting it together with another stitch. See Decrease (knitting). 

  • An amount by which a quantity is decreased. 

weaken

verb
  • To make weaker or less strong. 

  • To become weaker or less strong. 

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