Strange result using pg_dump gzip or split.

Hello,

I found strange result when I use pg_dump described on postgresql site:
http://www.postgresql.org/docs/9.3/static/backup-dump.html

I have a database with 30 gb data and decide to archive it, postgresql
is 9.3.5 x64_86, ext4 file system, kernel 3.14.18 Slackware 14.2
(current)

First I use gzip with : pg_dump logdb | gzip > log.sql.gz

After a few minute I have log.sql.gz with size 2 170 016 226
Well, that is strange and I dump database again with:

pg_dump logdb | split -b 1024m - log.sql

20 files is generated and I zip them with:

zip -r log.sql.zip logdir (because I move them in logdir)

file size is : 2 170 020 867

Almost the same, but if I check size in archives there is a huge
difference.

$ gzip -l log.sql.gz
compressed uncompressed ratio uncompressed_name
2170016226 3060688725 29.1% log_to.sql

and

$ unzip -v log.sql.zip
*** snip ***
-------- ------- --- -------
20240557909 2170020867 89% 20 files

Here is difference: with gzip I have 29.1% compress ratio and
uncompressed size is 3 060 688 725 which means 3 GB
and with zip I have 89% compress ratio and uncompressed size is 20 240
557 909 witch mean 20 GB. That is 7 times bigger.

My question is: Is there some special config params that is not
described in documentation here:
http://www.postgresql.org/docs/9.3/static/backup-dump.html
Or something need to be configured on my linux.

And most important question for me is: Did the database dump is corrupt
or not ?

Regards,

Hristo Simeonov