Optimizing Storage for Narrow Tables
If you have a table with very few columns but a very large number of rows, the three hidden metadata identity columns (INSERT_XID, DELETE_XID, ROW_ID) will consume a disproportionate amount of the disk space for the table.
In order to optimize compression of the hidden columns, load the table in a single COPY transaction where possible. If you load the table with multiple separate COPY commands, the INSERT_XID column will not compress well. You will need to perform a vacuum operation if you use multiple COPY commands, but it will not improve compression of INSERT_XID.