Postgresql FS is full but tables aren't so big - postgresql

Question

I want to check something regarding PostgreSQL performance during my app is running.
My app does the next things on 20 tables in a loop :
truncate table.
drop constraints on table
drop indexes on table
insert into local_table select * from remote_oracle_table
Recently I'm getting an error in this part
SQLERRM = could not extend file "base/16400/124810.23": wrote only 4096 of 8192 bytes at block
3092001
create constraints on table
create indexes on table.
This operation runs every night. Most of the tables are small 500M-2G but few tables are pretty big 24G-45G.
My wals and my data directory are on different fs. My data directory fs size is 400G. During this operation the data directory fs becomes full. However, after this operation 100G are freed which means that 300G are used from the 400g of the data directory fs. Something regarding those sizes doesn't seems ok.
When I check my database size:
mydb=# SELECT
mydb-# pg_database.datname,
mydb-# pg_size_pretty(pg_database_size(pg_database.datname)) AS size
mydb-# FROM pg_database;
datname | size
-----------+---------
template0 | 7265 kB
mydb | 246 GB
postgres | 568 MB
template1 | 7865 kB
(4 rows)
When I check all the tables in mydb database:
mydb-# relname as "Table",
mydb-# pg_size_pretty(pg_total_relation_size(relid)) As "Size",
mydb-# pg_size_pretty(pg_total_relation_size(relid) -
pg_relation_size(relid)) as "External Size"
mydb-# FROM pg_catalog.pg_statio_user_tables ORDER BY
pg_total_relation_size(relid) DESC;
Table | Size | External Size
-------------------+------------+---------------
table 1| 45 GB | 13 GB
table 2| 15 GB | 6330 MB
table 3| 9506 MB | 3800 MB
table 4| 7473 MB | 1838 MB
table 5| 7267 MB | 2652 MB
table 6| 5347 MB | 1701 MB
table 7| 3402 MB | 1377 MB
table 8| 3092 MB | 1318 MB
table 9| 2145 MB | 724 MB
table 10| 1804 MB | 381 MB
table 11 293 MB | 83 MB
table 12| 268 MB | 103 MB
table 13| 225 MB | 108 MB
table 14| 217 MB | 40 MB
table 15| 172 MB | 47 MB
table 16| 134 MB | 36 MB
table 17| 102 MB | 27 MB
table 18| 86 MB | 22 MB
.....
In the data directory the base directory`s size is 240G. I have 16G of ram in my machine.