Cube Index Size

Hi,

Cube code provided by postgres contrib folder. It uses the NDBOX structure.
On creating index, it's size increase at a high rate.

On inserting some tuple and creating indexes its behaviour is shown below.

1. When there is only one tuple
select pg_size_pretty(pg_relation_
size('cubtest')); //Table size without index
pg_size_pretty
----------------
8192 bytes
(1 row)

select pg_size_pretty(pg_total_relation_size('cubtest')); //Table size with
index
pg_size_pretty
----------------
16 kB
(1 row)

i.e. Index size in nearly 8kB

2. When tuples are 20,000

Table Size without index - 1.6 MB
Table Size with index - 11 MB
i.e. Index size is nearly 9.4 MB

3. When tuples are 5 lakh

Table Size without index - 40 MB
Table Size with index - 2117 MB
i.e. Index size is nearly 2077 MB ~ 2GB
It is taking nearly 20-25 min for creating index for 5 lakh tuples.

Can some one tell me why index is becoming so large?
How to compress or reduce its size?

Thanks
Nick