Most SSDs are already tiny. The 2.5" form factor is used just so that the drives can be physically mounted wherever a 2.5" HDD could be mounted. The drive is not full of chips (unless maybe it is a very high-capacity drive).
Here is a Samsung 860 family SSD with the case removed, for example...
Considering where some of those screw holes are, you can probably mount it. And depending on your setup, you could probably mount two of them in one 2.5" bay.