I remember in the past when there were stereo systems which can hold 3 or more disc to listen to many CD disc without changing them. I think they were not much bigger than a regular CD drive on cheaper more basic Home CD stereo systems which were top loading or has a slide-out tray like a PC's CD-ROM Disc drive. These Disc drives used disc slots like the Wii's Disc drive instead of those pop-out doors found on CD-ROM drives on PC. Most of the space on the drive is used up by the holes for holding each disc in place.
I think the extra space could be useful for adding a Memory card reader slot for MicroSD, Regular Size SD, and Sony Memory Stick cards. A lot of people still use Memory cards to copy, and view photos, video, and listen to music files which are stored on different types of Memory cards like MicroSD and SD cards.
Sony can also use the space to add a low power computer like the Chromecast, Roku, Raspberry Pi, and Android TV to the PS4 for using for watching Netflix, YouTube, and using Apps instead of using the PS4's console which uses more electricity, and runs louder because of the cooling fans.