Large pkey index on insert-only table

Hi all,
I have a suspiciously large index, and I could use a hand finding a root
cause for its size. This index supports the primary key for a closure
table that models threaded comments with columns `(id, ancestor_id,
depth)`. The primary key is composite: `(id, ancestor_id)`. The id
columns are varchars which are a bit long for identifiers, around 70
bytes. This table is insert-only: the application never performs updates
or deletes.

The table has grown to 200GB, and the unique index supporting the primary
key is nearly double that at around 360GB, which stood out to me as rather
large compared to the table itself. The index uses the default fillfactor
of 90. I would not anticipate very much bloat since updates and deletes
never occur on this table, and according to pg_stat_all_tables autovacuum
has been running regularly. I've used the btree bloat estimator from
https://github.com/ioguix/pgsql-bloat-estimation, and it estimates the
bloat percentage at 47%.

Any thoughts on why this may be, or where to go next to continue tracking
this down? Also, could the primary key column order `(id, ancestor_id)` vs
`(ancestor_id, id)` significantly affect the index size depending on the
column cardinalities? I appreciate your time and input, thanks!

--
Devin Ivy