If the column value on the b-tree index is monotonically increasing, then coalesce the index periodically.

I had the same experience few months ago. A 320 column table has 85 indexes and the table has 40 M rows and it's size 40 GB. Suddenly one index on a single date column had grown up to 8GB. The expected size is around 1.3 GB. This column is updated by the sysdate value. And heavy DML have been happening on the table 24x7. So the deleted space in the index leaf blocks were not reused effectively by oracle. I put a weekly cron job coalesces the index. After this the index size oscillates between 1.3 GB to 1.5GB.

Tamil