Insert only table and size of GIN index JSONB field.

Hi,

I started with empty table with index over
 custom_fields | jsonb

field

defined as:
    "idx_learners_custom_fields" gin (custom_fields)

Globally gin_pending_list_limit set to 2MB.

Database version is 9.5.2.

Now question:
If table populated with 1M records in single transaction then the final size of the GIN index is:
4265 MB

but after I performed reindex index idx_learners_custom_fields;

the index size had been reduced 15x to 295 MB.

Is this behavior expected?