RDS blue/green deployment is not decreasing the storage

The behavior you're experiencing with the RDS blue/green deployment not decreasing the storage as expected is likely due to a limitation in how Amazon RDS handles storage modifications.

When you create a blue/green deployment, the green environment is typically created as a copy of the blue (current) environment. In your case, even though you selected 200GB for the new storage size, the green instance maintained the maximum storage setting of 1300GB from your original instance.

RDS generally doesn't allow reducing allocated storage once it has been set to a higher value. This is true even when using blue/green deployments. The system is designed to prevent potential data loss that could occur if the storage size were reduced below the current data volume.

Your previous success in reducing storage from 100GB gp2 to 20GB gp3 might have been possible because you were changing the storage type (from gp2 to gp3) along with the size reduction. However, in the current scenario, you're trying to reduce the size within the same storage type (gp3), which is not typically allowed.

To address this issue and reduce your costs, you might need to consider alternative approaches:

1. Create a new RDS instance with the desired lower storage size and manually migrate your data.

2. Use database export and import tools to move your data to a new instance with the desired configuration.

3. Contact AWS support to see if they can assist with reducing the storage in your specific case.

Remember that when increasing allocated storage, it must be by at least 10% of the current size. However, decreasing allocated storage is not a standard operation and requires special handling as described above.

For future reference, it's important to carefully plan your initial storage allocation and maximum storage settings, as increasing these values can lead to situations where it becomes difficult to reduce them later.
Sources
Managing capacity automatically with Amazon RDS storage autoscaling - Amazon Relational Database Service
Amazon RDS Blue/Green Deployments support storage volume shrink - AWS
Increasing DB instance storage capacity - Amazon Relational Database Service
Reducing Allocated Storage for RDS Instance to Optimize Costs(Caused by Storage Autoscaling) | AWS re:Post