Trying to understand how can the pg_toast table be 90% of my entire DB?
It isn't "the", its "one of my". "One of my pg_toast" tables consumes 90% of my entire DB". Though its remotely possible you only have one it is not generally the case.
SELECT nspname ||'.'|| relname AS"relation", pg_size_pretty(pg_relation_size(C.oid))AS"size"FROM pg_class C LEFTJOIN pg_namespace N ON(N.oid = C.relnamespace)WHERE nspname NOTIN('pg_catalog','information_schema')ORDERBY pg_relation_size(C.oid)DESC LIMIT 20;
Is there anything I can do to save up some disk space?
Not outside of radical changes to your architecture or deleting records - though possibly marginal improvements could be made dependent upon the type of data that is being "toasted".