Downsized textures have less resolution, so of course they look worse.
Edinburgh airport is a great airport, but it’s also a relatively small one, so the developer can afford pushing resolution up while staying within a reasonable memory footprint compared to larger hubs.
If you look at general aviation airports like London Oxford, you’ll see a even higher level of detail.
An airport is like a scene. When that scene is loaded, it needs to be streamed into memory. All of it. Textures, 3D models, animations. The more memory it takes, the more it will affect performance. If it exceeds its allocation, it can absolutely tank performance, and it needs to share memory with everything around it, it doesn’t exist in a bubble.
Some developers will put more of their memory allocation into 3D details, and less in textures. Some will do more in textures and less 3D details, with varying balances depending on their art style, size of the project, more.
And yes, some developers will also just use crap textures because they’re not good, but that’s very rare among the reputable names.