Charles Parnot: Contentless SQLite FTS4 Tables for Large Immutable Documents:

"If you use SQlite FTS4 for indexing large immutable documents, you might want to consider contentless tables. With minor adjustments, the database could be 2 to 3-time smaller."

This is pretty neat and I'll most likely be doing this in a future release of Acorn, thanks to Charles' work here.