Kraid in the NES metroid was the same size as Samus, though I'm guessing that your referring to the Zero Mission Kraid there. Also, the SA-X didn't exist before Fusion, which is currently the final Metroid game chronologically.
As for Prime, since this is the final Metroid Prime game, Dark Samus could return to it's original form for the final battle.
By ozzo
Mission debriefing:
Now available