SQL & PL/SQL :: Index Compression Increasing The Size?

I had created a Primary key and wanted to compress as per my senior instructions.Below are my results the size increased after compression.

select compression from dba_indexes where index_name = 'TEST_IDX';
Compression
----------
DISABLED
select sum(blocks) no_of_blocks, (sum(blocks)*8192)/(1024*1024)size_MB

[code]....

We ran a compression on the primary key index TEST_IDX

ALTER INDEX SCOTT.TEST_IDX REBUILD INITRANS 15 TABLESPACE DATA_01 COMPRESS;
ANALYZE INDEX SCOTT.TEST_IDX VALIDATE STRUCTURE;

Now when i ran the below select statement:

select compression from dba_indexes where index_name = 'TEST_IDX';
Compression
----------
ENABLED
select sum(blocks) no_of_blocks, (sum(blocks)*8192)/(1024*1024)size_MB

[code]....

As you can see after compression the blocks and size has been increased, but i ran for many tables and other indexes, we observed the blocks and size was reduced by 50-70%, i am not sure why this happened to the index compression.