Is their any "default" compression involved or not? Does pg_dump talk to zlib during the dump process given that I have not specified any compression on the command line? (see above).
In general the easiest way to answer the question of whether something (well, a text file at least) is compressed or not is to attempt to open it in a text editor - if you can read the contents
its not compressed.
Reading the Description section at the following location provides the answer you seek - no, there is no compression by default.