I think you’re thinking too much into this.
If you have 1 storage group, the master node replicates its “replicatable” images to all storage nodes in the group.
If you have 2 storage nodes in one storage group, the images that are set to replicate will replicate from the master node to the other node. The space is not “divied” up. The master node will always be the node images captured are loaded to.
You could, I suppose, copy specific images from the master node and place it across other nodes so long as those images are not set to be replicated, but again, this does not increase your actual storage space. It just distributes it to the other nodes.
The simplest method, add more space to your master storage node.