Circular surgical stapler having staples with expandable crowns

A surgical stapler instrument includes a stapling head assembly that receives a plurality of staples. The staples are configured with features that allow the staples to expand after deployment so that an anastomosis created by the instrument can increase in size after forming. In some versions the staples expand automatically after deployment, and in other versions the staples expand in response to tissue forces imparted upon the staples after deployment. In some versions the staples are configured to be deployed in various patterns that promote expandability of the circular staple line.