I could imagine that this behaviour will be caused from the qvd-header and that the lineage-data or something similar increase and won't be dropped with the dropped table. I remember some postings about problems with them but I couldn't find them unless this old stuff: Re: QVD File Size in QV11.
During this process, I was also collecting the names of the processed files in a separate table that had no common fields with the main data table. At first, I was collecting all the file names for the whole run, and I was storing a single text file as a "log" at the end of the processing.
Later on, I changed the process of logging - instead of writing a single log at the end, I started writing smaller logs when each QVD file was stored, and I was dropping the log table immediately after. This time, the QVD file sizes didn't grow. In total, the summarized QVD size was about 5 times smaller compared to the first run with a single "log" table.
What keeps me puzzled is that the main data table had nothing in common with the log table, and yet managing a growing log was causing the growing size of the "main" QVD files, while managing many smaller logs helped keeping the "main" QVD sizes smaller...
Never stop learning...