Compiler optimisations aren't always designed to be semantically equivalent - for instance options which control floating point behaviour, like -ffast-math in g

Compiler optimisations aren't always designed to be semantically equivalent - for instance options which control floating point behaviour, like -ffast-math in gcc. That aside, sometimes compilers themselves have bugs as others have pointed out (I found one once involving a combination of python, boost, exceptions and the Intel C++ compiler - which only happened at -O3, not at -O2). Or you might not be able to use certain settings because it would be incompatible with some thirdparty library that you don't have the source to. Re: the size, the technical achievement is in managing all the content, how you interact with it and how it interacts with itself, etc. It's doing quite a lot of I/O to pull in the right assets at the right time - not to mention some computational geometry to figure out which are the right assets; running lots of AI for the creatures & NPCs; balancing memory usage between RAM & VRAM; simulating a night & day cycle, with weather too; rendering it all in (usually!) less than 30 milliseconds; and more besides. I certainly don't mean to play down the achievement of the artists involved, but don't underestimate the technical side either!