Hi Kalyan,
As Denis has mentioned, the index can grow to a fairly large size depending on how you are indexing your BO environment and how large the repository itself is.
If you are indexing ALL of the content for all document types, the index is going to be quite extensive.
You can set it to either just platform data, platform AND document metadata and full content. You can also choose which file types to perform indexing for.
All these options are found in CMC -> Applications -> Platform search.
Regards,
Siddhartha