You can either copy the whole app and call it�s frozen AXY app and ensure that no roles can update, or else you can copy the big table to another table in your app. However, if you do the latter and you have relationships, then you will lose all the relationships and their lookup and summary fields
To Copy the table (which conveniently will also copy your forms and reports, use the more button.
Once you have a secure copy of your data, then give that big table a careful haircut by making a report of what is to be purged out, checking it thrice, and then More .... Delete these rdr s, .... and an Are you sure ? And then you will have room for more records In your live table.