pg_dumpall size twist

I submitted a post about my pg_dumpall file being twice as big on one server compared to the other.
It turns out that one specific database called postgres is growing each time I perform a restore from the pg_dumpall files.
The database postgres has gone from 5.1MB to 10MB to 15MB to 20MB. No other database is growing like this. The pg_dumpall is using the -c flag to drop databases before restoring them.

When I do a restore I do this command: psql -U postgres -f alldb postgres.
Has it something to do with using postgres as the starting db?
I read in the documentation to start the restore with a starting point, and that postgres is a good place to start.

Marc