Nice job on the pics! You can definitely begin to appreciate the cargo space lost by the covers and foam space filler. It's really quite a deep well without those wasters, and could be a lot more space efficient in a more rectangular shape based on the unused space available under the car.
It's clear that GM once intended to put a temporary spare and jack kit in the car. Likely where the hidden storage is now is where the 120v charger kit was supposed to go. When they decided against the spare, the charger moved downstairs and we gained additional storage. It's great news for those that really want to carry around a spare. The rest of us just get extra storage.