Postgres: Reclaiming Space
PostgreSQL leaves dead data in the table; the space can be reused, but the files won't shrink (significantly).

The official method to reclaim space is VACUUM (FULL), but that rewrites the whole table, which will be unavailable for any access during that time. There are extensions called pg_squeeze and pg_repack which do the same thing with less disruption "behind the scenes".

All these methods have in common that they will require enough free space to create a copy of the table, so you probably won't get around increasing the storage space anyway.

Now for the good news:

If you run a plain VACUUM on the table, which is not disruptive, the wasted space can be reused. So your next attempt to load the data won't increase the size of the table.