Do not trust these max power consumption numbers given by the manufacturers to cover short peaks especially during startup or when recovering from some idle mode. Obviously it is not true as of the issues 
. Usually these numbers cover operation consumption when the drive is up and not in any power savings state (where it needs to recover from first).
Also note that RPi up to 3+ has USB 2.0 only. You will recognise that many USB-only powered cases ship with a split USB cable so that a second USB port can provide additional power. In case of RPi this doesn’t help as of the shared 1.2A (6W) max over all USB ports. On RPi 4 with USB 3.0 it’s the same: While USB 3.0 generally can provide much more power, PCs, notebooks and all such with USB 3.0 support do, RPi 4 still does not provide more than 6W, and it’s similar for most other SBCs.
While a single drive 2.5" USB case usually has no dedicated PSU or power in, a docking station or 3.5" case always has, where you can plug 2.5" drives as well.
An alternative is a USB pen drive, where I never heard of any power related issues. On RPi up to 3+ the USB speed is so highly limited that the extra speed of an SSD is completely lost anyway .