When adding to the library, it is populating the internal database with metadata whether that metadata is sourced locally or from the web. It does sound irrelevant to reducing the size of the internal database whether the metadata is stored locally at the time of scanning. As you say library import still imports the same metadata and thumbnails whether it is from local or internet.
My point was that you can access them through the file browser directly. Then it only loads the thumbnails on the fly as you browse the folders. You will lose out on the metadata and the nice presentation of the library, but to get around over populating the library this could be an option.
I have about 9,500 video files in my library and the database for that is 107MB. I have 40,000 songs in my audio library and that database is 60MB. I have overwritten OSMC’s default thumbnail cache size and increased it to full HD resolution, and despite that my thumbnail cache is only 2.07GB for the 33,158 files it contains. Note that this is a average of a whopping 62kb for each thumbnail.
But just for the sake of argument, if someone does have a storage size constraint for their thumbnail cache you can just move it to location with more room. This can be a SD card, USB drive, or network path, Kodi does not really care. If you don’t want a speed hit then you can use a SSD drive for this new location and you should get similar speed to the builtin eMMC.