Sudden multiplication of files within a single project?

I am experiecing a unexplainable multiplication of files within my project file. It had a size of ca. 240 MB and contained ca. 5.200 files a few days ago (at least that was the number given when re-publishing all files).

Then I copied over from another (test) project a Lockdown page with a stacks page underneath. When I exported my project the info given was: more than 33.000 files were being uploaded - which took ca. 30 minutes (with 10 MB/s upload speed). The overall size of the project is now 243 MB.

Does anybody have an idea what is happening here? How can I return to my smaller size of project? I copying over a page from another project a no go?

Puzzled…
Franz