Animal Well was coded by one guy, and it was ~35mb on release (I think it's above 100 at this point after a few updates, but still). The game is massive and pretty complex. And it's the size of an SNES ROM.
Dwarf Fortress has to be one of the most complex simulations ever created, developed by two brothers and given out for free for several decades. The game, prior to adding actual graphics, DF was ~100mb and the Steam version is still remarkably compact.
I am consistently amazed by people's ingenuity with this stuff.