Oddly? This is not odd at all.

It's been a while sincce I wrote code, but I'll try to remember. Basically disk size and ram size have no connection. Disk size is for already generated assets (maybe you need to remember how the planes look like, so you create assets for all the planes. Or you want to have textures for the scenery, or for the Lincoln monument, or whatever).

But then you need to load those resources into RAM to access them faster, because if you try to load them directly from disk, it's a lot slower. So some part of those 64GB of RAM is because you are loading some premade assets.

But aside from this, there's also dynamically generated data that you have no way of knowing about at the beginning of the program, so you can't prepare in advance and generate assets for it. Like say for example the player wants to begin flying the plane - he's gonna have some different inputs than any other player. Maybe he drives slower at the beginning, or goes a little to the right when he takes off. Or his destination will be completely different. You now need to remember his velocity, his position on the map, the direction of his flight, his altitude, his plane's weight and who knows what else, I'm not a pilot. All of this, you allocate memory dynamically, based on user changes, and this uses the RAM as well.

Not to mention - you can make a 1kb program that takes 64 GB of RAM. You just ask the operating system for that much memory. You don't even need to fully use it. It'll take you one line of code.

All this to say - nothing odd about the program being smaller than the RAM requirements. It can mean it's not optimized, but it can also mean it has a lot of dynamic calculations that it's doing and a lot of stuff it needs to remember (and in the case of a flight Sim this wouldn't surprise me).