Does Postgres compress data?

If I run this query:

select sum(length(html)) from Indexer.Pages;

I get:

15,680,005,116

However, if I type:

C:\Program Files\PostgreSQL>dir /s

I get:

Total Files Listed:
       5528 File(s)  7,414,385,333 bytes
        575 Dir(s)  43,146,137,600 bytes free

So all the Postgres data on disk is a little over 7 gigs, however the
total sum of bytes in the HTML column of the Pages table is over 15
gigs.

Is PG compressing this data?  I'm curious as I was considering
converting this column to a byte array and gzip'ing the data to save
space, however if PG is already doing this for me, then I'm not going
to bother.  Thanks!

Mike