The growing index directory will be due to
Case1:- Content is added or modified continuously in the workspace. This operation likely results in creating a new segment (the folders in the index folder) with new documents in it.
Case2:- The segment created above needs to be merged. This process occurs asynchronously and triggered by modifications to the repository. The segment merge will run until it completes and still run when no more changes are performed.
 *INFO * IndexMerger: merged 250 documents in 724 ms into _21f. (IndexMerger.java,.....
Merging index segments may temporarily use up to three times the initial index size. As an example
Summation of above three now results in the new index size of approximately 30 GB of disk space. However in a final step the old index segments and the non-compound files are deleted & reduce the disk usage shrinks to 10 GB.
Reduce number of nodes by removing nodes that are not required like completed workflow instances, audit log etc.. The article that helps are
Additionally re-indexing the workspace might also reduce the index size because the index will not free disk space immediately when a node is deleted. This only happens when the affected index segment that contained the node is merged.