jffs2 vs ubifs vs ext4 space-efficiency question

[Adam Holt]

Any ideas why Release 13.2.6 placed on ext4 instead of jffs2 appears to
almost double in size?

Starting with 13.2.6 on XO-1 on jffs2 per usual, "du -s /* | sort -nr"
yields these contents for a stock/vanilla XO-1:

   1070992 /usr
   239060  /home
   117623  /var
   18780   /etc
   15382   /boot
   8735    /versions
   1032    /run
   8        /tmp
   ...

Naively summing up the above numbers gives a figure of almost 1.5GB, and
yet we all know the XO-1 is limited to 1.0GB :-)  Is jffs2 somehow
compressing the above files by about 2X, which are clearly contained within
769MB as shown in "df -hT" below?

   Filesystem  Type  Size  Used  Avail   Use% Mounted on
   mtd0           jffs2  1.0G  769M  256M  76%  /
   ...

Basic tests putting 13.2.6 on an SD card with ext4 show a rough doubling in
size with the same files (du's ~1.5GB of files indeed sum to about 1.5GB
when listed using df), which I presume indicates ext4 dispenses with
compression, unlike the more space-efficient to jffs2 (and presumably
ubifs) etc designed for flash/SD devices ?

Apologies for my naivete: Would any ext4 partitioning options (or ubifs
instead) offer any functionality similar to this apparently huge
compression win offered by jffs2?  (Given I'm told jffs2 is not appropriate
for the larger SD cards I'm experimenting with, even before we get to
wear-leveling questions!)
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