The SQL server database size is 609GB and one of the index (clustered)size is 598 GB and the particular table size is 604GB. whether the index contains data? why the index is this much big?
A clustered index is the table.
Clustered indexes sort and store the data rows in the table or view based on their key values. These are the columns included in the index definition. There can be only one clustered index per table, because the data rows themselves can be stored in only one order.