Query regarding EXPLAIN (ANALYZE,BUFFERS)

Hi

I wanted to query top 20 rows by joining two tables, one table having
around 1 lac rows and other table having 5 lac rows. Since I am using ORDER
BY in the query so I created compound index with the columns being used in
ORDER BY. Initially index size was 939 MB.

Then I ran EXPLAIN(ANALYZE,BUFFERS) for this query which took around 20
secs as it was not using the compound index for this query. So I drop this
index and created again. The index size now got reduced to 559 MB.

After this if I ran the EXPLAIN(ANALYZE,BUFFERS) for this query it was
using the index and took only 5 secs.

Can you please explain how the index size got reduced after recreating it
and how the query started using the index after recreating?

Thanks and Regards
Subramaniam