Non-Miniature Use and Decimation
As it states in the descriptions these files are prepared specifically for miniature use. They absolutely can be scaled up to statue sizes (try it with the free one!) and will look great with a nice paint job, which is where all the magic happens anyway, but understand that printing them at huge 200mm+ sizes is not what they are intended or prepared for so don't expect them to look as good raw at those sizes.
Decimation is simply the optimization of the geometry, reducing the amount of polygons without affecting the overall detail of the model, resulting in a smaller file size and easier to work with file. Every 3D artist must do it or every time you download a mini it'd be hundreds of mb! Here is a direct comparison of one of my miniature files, unoptimized on the left, optimized on the right. (https://i.imgur.com/4FoYwJm.jpeg) That's all it does, reduce the file size.