Postgres: my "data/backup" directory is way too large
Here's a du -sh * output of a typical PostgreSQL 9.3 data directory:


4.0K    PG_VERSION
59G     base
47M     global
20M     pg_clog
28K     pg_multixact
80K     pg_notify
4.0K    pg_serial
4.0K    pg_snapshots
4.0K    pg_stat
368K    pg_stat_tmp
168K    pg_subtrans
4.0K    pg_tblspc
4.0K    pg_twophase
1.1G    pg_xlog
4.0K    postmaster.opts
4.0K    postmaster.pid


Notice that there's no backup directory: a PG instance does not need or create such a directory. It had to be created by a person or a script that is not part of PostgreSQL itself.

Since you mention pg_xlog, if the files in this directory follow the naming convention of WAL files (24 numbers in hexadecimal in a 3x8 layout) found in pg_xlog, it's plausible that they're WAL files copied for continuous archiving. Check the archive_command in postgresql.conf.

In this case, the name backup for the target directory make sense, but its location doesn't. Such backups are useful for disaster recovery when the data directory has disappeared, so obviously the data directory itself is the last place where you want such backups.