No question that on the outside the Rivian is a much larger SUV. As you point out the R1S is 12.1" taller, 0.6" wider and 2.6" longer but the Gravity has more interior volume due to where Lucid and Rivian chose to focus their design effort.
Rivian focused its effort in two areas; software and interior/exterior design. Mostly on the interior since the R1 is rather box like on the outside. Rivian interior design and ergonomics are top notch and match their intended use. I have been very impressed during my test drives. Likewise the Rivian software is excellent and again matches the intended use of the R1S. The many R1 owners here that praise Rivian software are spot on. Rivian did not focus on battery, inverter, drivetrain, chasis, body control, stability control but chose to buy off the shelf from teir one suppliers. This had the advantage of reducing engineering and shortening development time for Rivian but it comes at the cost of how all the pecies fit together. Without controlling the design, the pieces don't fit together compactly and the result is less interal space and a larger outside dimension.
Lucid choose to focus on battery, drivetrain, suspension, vehicle dynamics and interior design, largely ignoring software. Like Rivian, Lucid nailed it for interior design, materials and ergonomics. Lucid also took advantage of owning the drivetrain, battery, inverter and chasis design to optimize efficiency and also to optimze how the pieces fit together to minimize space. This is how Lucid pulls off the magic of more interior space in a smaller exterior dimesnion. It is also why Lucid is leading in EV efficiency. No need to comment here on the disadvantage of Lucid's software approach though.