SQL Server 2005: Index bigger than data stored

I created 1 database with 2 file groups: 1 primary and 1 index.

• Primary file group includes 1 data file (*.mdf): store all tables
• Index file group includes 1 index file (*.ndf): store all indexes

Most of indexes are non-clustered indexes

After a short time using the database, the data file is 2GB but the index file is 12 GB. I do not know what problem happened in my database.

I have some questions:

1. How do I reduce the size of the index file?
2. How do I know what is stored in the index file?
3. How do I trace all impacts to the index file?
4. How do I limit size growing of index file?