unexpected external sort Disk

Hello,

I have table with 37 million entries the whole table has a size of  2.3 GB

Although I have set the work_mem to 10 GB

I see the an unexpected external sort Disk in Explain Analyze for around 650MB of data

 EXPLAIN ANALYZE  SELECT application_id, price_tier FROM application_prices order by application_id, created_at;

                                                               QUERY PLAN

----------------------------------------------------------------------------------------------------------------------------------------
 Sort  (cost=5284625.89..5378196.50 rows=37428244 width=8) (actual time=36972.658..40618.161 rows=37428244 loops=1)
   Sort Key: application_id, created_at
   Sort Method: external sort  Disk: 658568kB
   ->  Seq Scan on application_prices  (cost=0.00..576597.44 rows=37428244 width=8) (actual time=0.012..6259.923
rows=37428244loops=1) 
 Total runtime: 42999.882 ms
(5 rows)

Has anyone an idea what I'm missing ?

Thanks

Manuel