Since you changed the volume size, there is some logic in the compaction calculation that works a little odd. It looks for “small” files, which by default is set to volume size / 5.
Because the volumes cannot be filled to the exact byte, the previous ones were less than 50MiB, meaning less than 1/5th of the new volume size. Because there are now a lot of small files, they are compacted.
The logic for compacting is to proceed with generating new volumes and marking old ones for deletion. This makes it a bit robust, as an interrupted backup can then know which files to delete, even if the compact process did not completed.
At the end of the compact process, it will then delete all volumes that are no longer needed. This design is intended to be robust, but has the downside that you will need more space for a short period.
tl;dr: It will delete the files at then end of the compact step.