If the storage capacity of a Cloud SQL instance is larger than your application needs, then you can manually reduce, or...

If the storage capacity of a Cloud SQL instance is larger than your application needs, then you can manually reduce, or shrink, your storage capacity to a smaller size.

Depending on underlying disk size, storage shrink operations might incur considerable downtime. If your instance requires limited downtime, rather than using storage shrink capabilities, we recommend migrating your data to a new, smaller instance using Database Migration Service.

For more information, see About storage shrink.