Tom was exactly right.
I was trying to restore the dump file into an already created table
structure that did have three foreign key constraints. I removed the
primary key constraint to speed up the load, but was not aware of the
memory usage of the foreign keys.
I dropped the table and ran the pg_restore again, and it is running
currently and behaving nicely at constant 13Mb of memory. It will
probably take some time, but I think it is working fine.
Thanks for the help.