How do I find RDS Snapshot Incremental sizes for proper cost estimation ?

I understand that:

The first snapshot of a DB instance contains the data for the full DB instance. Subsequent snapshots of the same DB instance are incremental, which means that only the data that has changed after your most recent snapshot is saved.

Customer is taking 1 backup per day (aging out after 7 days). All of the snapshot are reported as full sizes. How can we identify the incremental sizes of each snapshot? This would provide additional details into how billing is done for backup storage more than the database size itself.