It can't be the game only. Check for the running add-ons in the browser, applications and processes, and the resources they are consuming. There must other things consuming a good part of the RAM.
Also, your activity monitoring software seems to be a bit confusing. For example, the difference 644 MB vs 1.7 GB and 17.4 MB vs 2.2 GB. The two screenshots does not seem to correspond to the same system.