Eh, maybe. The suitcase itself is also, more simply, literally just something designed to be a reasonable amount of stuff that a human could carry. Many early portable PCs (e.g. Osborne) targeted a suitcase form factor just because it was a reasonable form factor for people to carry around.

And the comparison is something colloquially convenient. "The device was the size of a suitcase" is just more illustrative than "the device was 50 liters in volume"