Given previous versions of this enclosure (with I believe the same exterior dimensions) managed to fit the computer plus two 2.5” old school hard disks, Apple c

Given previous versions of this enclosure (with I believe the same exterior dimensions) managed to fit the computer plus two 2.5” old school hard disks, Apple clearly could engineer the interior of this device to fit a much, much smaller m2 SSD. This is a question of engineering choices, not physical space limitations. In this case the engineering choice was probably driven at least in part by the desire to take advantage of the T2’s line rate encryption features, which so far has never been supported on removable/upgradeable storage mediums (at least as far as I am aware - the T2 drive encryption is a separate feature from FileVault, which is supported on removable media). Even on the iMac Pro which technically has removable flash storage, to my knowledge no one has yet managed to produce a T2 compatible after market third party upgrade. Thankfully thunderbolt 3 removes much of the advantage internal storage media traditionally enjoys, at least in performance terms, so there are reasonable upgrade paths.