Table Physical Size Surge

create table planner.unit_flow_section
(
    id                  serial,
    title               text,
    items               jsonb[]
);

There is an operation to remove a jsonb element from the items array.

I have an operation which takes a lot of time.

UPDATE planner.unit_flow_section SET items = array_remove(items, '{"a":"b"}'::JSONB) where id = 34;
Few days back we encountered a surge in Database size and within an hour 10GB of data was used up.
After debugging we found this table size increased upto 12GB. We executed this query to get the table size.
SELECT pg_size_pretty( pg_total_relation_size('planner.unit_flow_section') );
But when I create a copy of the table then that table size was only 66MB.
We are using this version of Postgresql:
PostgreSQL 10.6 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-11), 64-bit
We are not able to figure out the reason of this.