Can anyone explain to me the "magic" word "optimization" based on my question?

What I'm talking about is how did developers "optimize" games with very high system requirements (like GTA IV and V) to run on a 512 MB RAM Xbox 360? I can't grasp this as you would never be able to run those on a 512 MB RAM PC. How is it possible that they managed to make those run on a console but not a PC?