I'm not sure how that translates when you add it to a library, I'll have to investigate that.

The editor will create a subfolder and copy the obj-file in it. It copies the mtl-file and renames it to the same name as the obj-file. The obj-file is changed to reference to the the mtl-file. It copies the texture and renames it to have the name of the obj-file an underscore and its original name. The mtl-file is changed to reference to this texture.

If you add multiple objects you will get the same mtl-file and texture multiple times with different names in the subfolder of each obj.

In my case with three doors with a white panel and three doors with a wooden panel I have the following sizes:
sh3f: 101.543 Bytes -> 92.331 Bytes
sh3d: 303.417 Bytes -> 103.476 Bytes

If you are interested you can download the files.
WaHeModDoors.sh3f
WaHeModDoors_common.sh3f
Version_14_wood.sh3d
Version_14_wood_common.sh3d