Understanding where pg_toast is coming from
You can use this query to find out all tables with TOAST tables:

SELECT oid::regclass,
       reltoastrelid::regclass,
       pg_relation_size(reltoastrelid) AS toast_size
FROM pg_class
WHERE relkind = 'r'
  AND reltoastrelid &lt;&gt; 0
ORDER BY 3 DESC;


To find out which of the columns in the table consumes most space, you could try a query like

SELECT sum(length(col1)) AS col1_size,
       sum(length(col2)) AS col2_site
FROM some_table;


Here col1 and col2 would be text, varchar, char or bytea columns, which are usually the largest ones. With other data types like jsonb you could cast to text to get an estimate.