PostgreSQL drop table command is not reducing DB size

6586KB is about the size of an "empty" (freshly created) database. If the tables you dropped were very small or empty, dropping them will not reduce the size by much if any.

When I drop a populated large table, the reduction in database size (as seen by psql's "\l+" command) is reflected near instantaneously, with no need for a vacuum or a checkpoint.

VACUUM (or VACUUM FULL) is hardly useful in this case, since it only reclaims space from within tables.

Well, there are entries in catalog tables that would leave dead tuples behind after dropping a table. So the database can occupy slightly more space after a table has been created and dropped again.

To get a db down to minimum size again, run the client tool vacuumdb with the --full option:

vacuumdb -f mydb

On Postgres 9.0 or later, this also rewrites indices in prestine condition. Details in the Postgres Wiki: http://wiki.postgresql.org/wiki/VACUUM_FULL

That's how Postgres measures db size: with dedicated object size functions:

SELECT pg_size_pretty(pg_database_size(mydb));

Use truncate, understand the warning about MVCC first

TRUNCATE quickly removes all rows from a set of tables. It has the same effect as an unqualified DELETE on each table, but since it does not actually scan the tables it is faster. Furthermore, it reclaims disk space immediately, rather than requiring a subsequent VACUUM operation. This is most useful on large tables.

http://www.postgresql.org/docs/9.1/static/sql-truncate.html