What version are you using? What is your usage pattern for insert/update/deletes? If sometimes the JSON data gets too big and the data is moved from in-line storage to TOASTED, then that would be opening up gaps. Or if you are doing deletes. Perhaps adjusting your fillfactor and/or TOAST_TUPLE_TARGET would influence that behavior in the direction you want to go. As best I understand though if you are doing deletes, you won't be able to prevent those gaps from showing up and being re-used. I don't believe there is any way to influence Postgres to append-only the tuples and pages.
