Why are Mac requirements for games so much higher than Windows?
The problem is you are looking at a game, Medieval II: Total War, that was released for Windows in November 2006 (last patch released in 2007 as far as I can tell) and for OS X in January 2016. The Windows version runs on 2000/XP (Windows 2000 was originally released in 1999), whereas the minimum for Mac is OS X 10.10.5, which came out a bit under two years ago. In other words, the game was ported to Mac very recently and therefore requires a modern computer to run it. I am not sure why the disk storage requirement is so much higher; I suspect it's either a typo or the assets in the Mac version have been significantly enhanced.

If you compare Total War: Rome 2, another game in the same series but one that was only released a year apart on the two OSes, you will see system requirements that are much more similar. In fact, the Mac version actually requires less hard drive space. It does require more RAM, but I would guess that's more to do with the underlying OS than anything else.

I looked at some of the games in my Steam library to further confirm that system requirements are not inherently higher for games on Mac. SOMA actually has identical requirements for the two OSes. Marvel Heroes 2016 is very similar for both. The Witcher: Enhanced Edition has slightly higher requirements for Mac than Windows, especially in RAM, but again it runs on much older Windows OSes (XP) than OS X (10.8 minimum). Team Fortress 2 has nearly identical requirements.

Ultimately it comes down to several things: release time frame (again, comparing a game that came out in 2006 with a port that came out in 2016 is not going to yield similar results); the skill set of the developers involved (a studio who may not be as familiar with Mac may not be able to optimize a game as well as one who is, e.g. Aspyr); whether the game is wrapped in an emulation layer (such as Cider or Wine, which some large publishers do as an alternative to having a company like Aspyr port the game to a native Mac version); and ultimately, how much the publisher cares about the quality of the game and spends time (and of course money) optimizing the game for the target platform.