Well if you want to bring compression into the picture you're back down to around 14-20 bytes which again is most likely chump change in the big picture. :)

$ l {foo,bar}.yml*
-rw-r--r--  1 fletch  fletch  275 Jul 21 11:28 bar.yml
-rw-r--r--  1 fletch  fletch   91 Jul 21 11:28 bar.yml.bz2
-rw-r--r--  1 fletch  fletch   80 Jul 21 11:29 bar.yml.gz
-rw-r--r--  1 fletch  fletch  110 Jul 21 11:29 foo.yml
-rw-r--r--  1 fletch  fletch   77 Jul 21 11:29 foo.yml.bz2
-rw-r--r--  1 fletch  fletch   61 Jul 21 11:29 foo.yml.gz
$ for i in {foo,bar}.yml ; { print $i ; cat $i }
foo.yml
---
a: [1, [2, [3, [4, [5, [6, [7, [8, [9, [10]]]]]]]]]]
b: [1, [2, [3, [4, [5, [6, [7, [8, [9, [10]]]]]]]]]]
bar.yml
--- 
a: 
- 1
- - 2
  - - 3
    - - 4
      - - 5
        - - 6
          - - 7
            - - 8
              - - 9
                - - 10
b: 
- 1
- - 2
  - - 3
    - - 4
      - - 5
        - - 6
          - - 7
            - - 8
              - - 9
                - - 10
[download]

Of course if you're compressing before tossing blobs into your DB you've lost at least immediate readability (but then on the other other hand that's just a short helper utility from being back hyoomon readable nicely indented).

As another suggestion, if you've got a (relatively) small class of input data you might just roll your own serialize routine which spits out a more compact YAML representation rather than using one of the off-the-shelf modules.