There are a couple things you might be able to do to trade performance for index size. For example, an integer (int) field uses less space than a trie integer (tint), but range queries will be slower when using an int.

To make major reductions in your index, you will almost certainly need to look more closely at the fields you are using.

• Are you using a lot of stored fields? If so, try removing the stored fields from the index and query your database for the necessary data once you've got the results back from Solr.
• Add omitNorms="true" to text fields that don't need length normalization
• Add omitPositions="true" to text fields that don't require phrase matching
• Special fields, like NGrams, can take up a lot of space
• Are you removing stop words from text fields?