Florian
4 months ago
The files are indeed very large because 16k resolution is necessary to capture all the detail and dynamic range for high-quality reflections and lighting.
Regarding the viewport mapping, please keep in mind that an HDRI is a 360-degree spherical map that wraps around your entire scene. While the flat preview image on the product page shows the entire stretched sphere, a 3D camera in Blender only captures a small cropped portion of that environment depending on your focal length. This is the standard way environment maps work in all 3D software.
I hope this helps to clarify how spherical mapping and camera framing work!