Is there a reason to store the data twice? That's what you're doing since you have a clustered index on the ID column, and a non-clustered index on every other column.

Do you really need an ID column on this table? If so, does it really need to be the clustered index key?

The other issue, of the index getting larger than the table, is probably due to fragmentation. You might want to look into that and see if you can reasonably defragment it. Might need to adjust the fill factor to make it do fewer page splits. That's applicable whether it stays as a non-clustered index or becomes the clustered index.