Blender export stl file is larger than obj file

stl file size is fundamentally proportional to the number of faces in your model (after implicit triangulation), so having a file that big means that your mesh has a a lot of faces. It's either detailed, or many faces are redundant (unnecessary subdivision).

you can also use the depth format
https://krpano.com/docu/depth-file-format

I would like to know more about the difference between these two types of files,
and why one should use one or the other in specific.
What is the MTL file that is generated next to the OBJ in Everpano?
Can you get the same result, using any of the three files mentioned? OBJ, STL or depth_file-format?