Why are only some savegames compressed?

I've just noticed that some .ck2 savegames take more or less 10x of some others, during the same match (~45MB vs ~4.5MB).

I've tried to compress them, and the bigger one compresses down to a little smaller than the smaller one (~4MB), while the smaller one stays the same. This is a very strong indication that the smaller one is compressed.

Now… why are some of them compressed and some others aren't. Which causes that? Is that a "quick save" vs. "regular save" behaviour?

Can I somehow compress the uncompressed ones, so that they are still directly readable by the game?

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I've noticed that the uncompressed ones are text files, while the compressed ones start with the code "PK", so I've listed its content using unzip and it gave a somewhat promising result:

-rw-------  1 lohoris  wheel  46122334 Dec 26 16:25 Eire1127_08_25.ck2
-rw-------  1 lohoris  wheel   4498940 Dec 26 16:25 Eire1125_08_17.ck2
lohoris@Pirenne:/tmp$ unzip -l Eire1125_08_17.ck2
Archive:  Eire1125_08_17.ck2
  Length     Date   Time    Name
 --------    ----   ----    ----
 45466533  00-00-80 00:00   Eire1125_08_17.ck2
      109  00-00-80 00:00   meta
 --------                   -------
 45466642                   2 files

However I wasn't able to uncompress them from command line:

lohoris@Pirenne:/tmp$ unzip Eire1125_08_17.ck2
Archive:  Eire1125_08_17.ck2
   skipping: Eire1125_08_17.ck2      need PK compat. v4.6 (can do v2.1)
   skipping: meta                    need PK compat. v4.6 (can do v2.1)

At this point I got lost trying to uncompress it, so before investigating further I'll just wait to see if someone already has some answer.

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So, let's recap, questions:

• in which circumstances does the game sometimes create compressed files, sometimes uncompressed?
• can I turn the uncompressed ones into compressed, so that the game natively reads them correctly (obviously I can compress and decompress them, that's not what I'm asking)