downloaded tar.gz turns larger

tar.gz downloaded from Files tab is larger than the original file.
Test code:

dd if=/dev/zero of=output10.dat bs=1M count=10
dd if=/dev/zero of=output11.dat bs=1M count=11
tar -czf test.tar.gz output10.dat output11.dat
ls -la test.tar.gz

Output: 21614 Sep 6 09:36 test.tar.gz
i.e. 21.107421875KB.

Local PC:

ls -la test.tar.gz

22026240 Sep 6 17:37 test.tar.gz
i.e. 21.005859375MB.
What's going on?
It do takes longer to download for my real repo.tar.gz, and the file on local takes more space. The good news is the extracted files remain the same, verified by diff -r.

Different file systems report sizes differently.

I don't think that's the cause. output10.dat both reported 10MB. Tested on 16MB, 17, 18, 19, 20MB .dat files.

tar.gz extension is the buggy part.
My PC is using Linux Mint:
Linux xx-xxx 4.4.0-45-generic #66-Ubuntu SMP Wed Oct 19 14:12:37 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux

If you download a file from the "Files" tab, it's possible that your browser is recognising it as a gzipped file (I think we would set the MIME type appropriately, and even if not, there's the file extension) -- so maybe it's decompressing it?

Yes, that's the cause! Thank you for pointing out.
I just compress my repo as gz and cp to .dat format.

-rw-rw-r-- 1 xxx registered_users 43602629 Sep 7 11:10 test.dat
-rw-rw-r-- 1 xxx registered_users 43602629 Sep 7 11:10 test.tar.gz

-rw-r--r-- 1 x x 43602629 Sep 7 19:11 test.dat
-rw-r--r-- 1 x x 123873280 Sep 7 19:11 test.tar.gz

Archive Manager says all contents inside test.tar.gz are 123.4MB. It is decompressed by Chrome.

Excellent, glad we were able to work that one out.