Hi!
The index table search performance is based on the separate table rows for each possible keyword, with it’s occurence values. While it is much much faster to look up as opposed to the regular content tables, it’s size is bigger because of that.
I don’t think there is a way as of now to use a separate database via WordPress for the plugin tables, so I think the best way to reduce the table size is by minimizing the number of rows – thus the number of keywords.
To do that, I recommend checking the following configurations on the index table options
– Make sure that only the required post types are selected – I assume you already have that, but it can make a difference
– Choose only the fields, that are neccessary for the search – custom fields, taxonomy terms can increase the table size drastically. Deselcting some of those could reduce the table size.
– Increasing the min. word length – in case of huge datasets, short words might not be as relevant. Depending on your scenario, ignoring words below this length could reduce the size further.
– Using the stop-words – this is also recommended, especially with such big datasets. You can even add more words related to your content, that are frequent, but might not be as relevant.
After making all the changes, you will have to re-create the index table of course.
I believe these are the best tips to reduce the overall table size drastically.