The build, starting with the SRAM bits
The Rival 22 short cage RD comes in at only 179g. That's only 1g heavier than the Force 22 with a carbon cage.
It's also about half way between Dura-Ace and Ultegra weight, despite being third-tier.
You might be wondering, how is this possible?
First, lets start off with what makes a SRAM rear derailleur different from the rest. Yes, it has 1:1 actuation and that funny looking cam, but there are significant differences in the way a SRAM RD is constructed.
Several bicycle mechanic authorities claim that Shimano invented the modern derailer, which all modern derailers are patterned off of. This isn't true. A SRAM RD actually has more in common with a Suntour derailer than a Shimano one, because it uses a single sprung pivot design. Modern Shimano, Campy and Microshift road rears have a sprung b-knuckle (copied from Simplex), where the RD attaches to the hanger which helps a derailer adapt to a wide range of cog sizes. A single sprung design on the other hand does not have a spring there, it is essentially rigid in operation, and moves out of the way when removing the wheel. The path of the jockey wheel is rigidly defined by the parallelogram geometry. It's also popular in MTB because it helps reduce chain bounce, since the pulley cage isn't floating. That's another reason SRAM shares another design feature with some Suntour designs, it features an upper jockey that is concentric with the lower pivot, because otherwise front shifting would change the location of the pulley (and this is why 1x has lots off off-set instead of a slant parallelogram).
Basically, it's simpler, and lighter, because they didn't copy Shimano (they copied Suntour)
SRAM also uses some interesting choices of materials on the Rival RD. It has an alloy inner plate, like what is traditionally found on Ultegra derailers. It also has an alloy fixing bolt, which saves a significant amount of weight over a steel one, although after market ones have had a reputation for cracking and the bolt is softer and easier to strip. This is why Shimano uses steel fixing bolts, or the funny looking 2-material bolt on Dura-Ace. SRAM also uses a plastic bottom knuckle for the Rival, that's right, plastic. Shimano used to do the Same on older Sora and Acera, and still makes Tourney with plastic knuckles. However, it's something Shimano has phased out of Sora, while Shimano copied the Simplex dual sprung pivots, they didn't share Simplex's affinity for plastic derailers. Plastic is cheap, but it's also light. For things that won't require lots of strength, but need to take a certain shape, plastic is about as light as carbon fiber. SRAM also uses aluminum for the cages, because the truth is, aluminum really doesn't cost much more than steel. It's not all weight-saving corner-cutting for SRAM though. SRAM put ball bearings in their jockeys (13g ea), which makes them higher quality than bushings, but with a thick grease and an undemanding job, they don't necessarily spin better. It does add weight and cost though.
Shimano trickles down technology, but not weight, weight is a key defining factor for product differentiation as well as perceived quality of construction. SRAM is willing to keep the weight weenism going in the lower tiers, and will use use whatever mix of cheap and expensive processes based on how much it can help reduce weight.
And the last bit, why is the Rival 22 RD only 1g heavier than the Force 22 RD, even though the Force 22 RD has a carbon cage? That requires a look at Force 10. It's basically the exact same thing as the Force 22. Same weight, same construction, same design, different color. Moving over to the Rival side, Rival 10 was 188g, 10g heavier than Force because of the metal cage. However, unlike Force, Rival 22 was redesigned, with a new front plate, and possibly other changes, reducing weight by 9g. It's very likely a Rival 22 RD with a Force cage would be ~10g lighter than a Force 22. However, because Rival 22 is only 1g heavier, they had to cripple it, by intentionally painting it with ugly glitter silver paint (like older Sora) even though the shifters use white paint.
So, Rival 22 RD is quite competitive with Force 22 because it got a revamp coming from 10 speeds, unlike Force.