In a help ticket, I observed there was a dramatic increase of the history store file size with the increase of the history window. That is expected. However, after we decrease the history window, the size of the history store file didn't decrease correspondingly.

Whether we could decrease the file size of a WiredTiger file depends on if there were contents still needed at the end of the file. Therefore, we often need to compact the file to free the available spaces in the file. In the case of history store file, I would imagine the most content should be obsolete after we have decreased the history store window and it is unlikely we cannot decrease the file size.

We need to understand why this can happen.