It believe it depends how you have configured your insights DB, in our case we’ve set it to take a full backup from our production server every 3 hours and restore the backup onto the server we’ve set up the database for insights on as you never run ETL against your production server.
If you’ve enable tlog shipping then check your frequency.
Your main DB should be fully replicated to your insights DB so should “technically” replicate whats in there and no more. It really shouldn’t be any bigger I don’t believe.
The best person to confirm though would probably be @Aram