This is somewhat contradictory when we consider that, despite the SA-X assuming the form of Samus with the Varia Suit, Samus in
Fusion states that
the SA-X is her with her strongest abilities, and
Dread's opening parrots this by stating that
the SA-X was herself but at full strength. Adam also states that the
SA-X is mimicking Samus at full power. So yeah, it's weird that the SA-X would assume the Varia Suit and not the Gravity Suit despite the claims by Adam and Samus herself of the SA-X being at her full power
SM self, but that's the way the cookie crumbles. I feel like they just went with the Varia Suit because it's the more iconic suit considering it's used all over stuff like box art and promotional material.