Export-import part of kb much harder than export-import entire kb?

My knowledge base has ~20000 rems, export entire kb (raw backup JSON) the size is around 10 MB.
Export one document in it (raw backup JSON) the size of the zip is 200 Mb. It’s a document with many nested rems, many links, tags to other rems. But still, this is odd.

Why is it so much harder to export and import part of knowledge base than entire knowledge base? It took like 5 minutes to import my 20000 rems kb to a new one, but the app failed to import one document in it after 4 hours and got stuck in a blank window.

I reinstalled my computer and the 3000+ notes I exported before can’t be re-imported! I just get a blank page!