I'm sure if you posted the EXPLAIN here, someone might be able to figure out why it did that. Were you able to test the exact same query on a different database?
I was going to, but something else got in the way and I ended up not running the query again.
Over the years I've had my share of complex queries with subqueries and aggregates, both for class and for my own projects, but never have I encountered 70MB exploding into multiple gigabytes (I don't even know what its final size would have been). I guess I could have used EXPLAIN and dug into it, but never having had this before I figured it was SQLite not being made for it.
