Hi,
Userexit is enabled in our environment. I read in the document that logs in the active directory needs to be manually purged once they get moved in the other directory. But why are they automatically getting purged in our environment?
Also help me know, why are the logs that got archived are not of the same size? What I understand is, log files moves when they get full; in that case, I should see all the logs files in the archived folder to be of same size...

If you enable USEREXIT, it is the userexit program responsibility to delete (or not) the archived logs.

...and also in other cases, e.g. after a backup, or when the database is deactivated.

If userexit is enabled, db2 should reuse the logs in the active log directory by renaming them. Let's say, log A gets archived by userexit. When log A is no longer active (full logs are archived when they're still active) and DB2 needs to allocate a new log B, it should rename log A to log B. The archived logs can be of different size if they get truncated (at the end of an online backup, archive log command or when the database is deactivated as n_i mentioned). Truncated logs used to be deleted in older version, but they are now extended and reused.