SRAM came up with a super expensive cassette machined out of a single lump of steel (X Dome). On the whole this is cheaper for them to make if it’s 10% smaller and they can charge more if it’s lighter. The matching freehub body (xD Driver) gets them under the former 11t limit on the Shimano freehub. It shut out Shimano for a little while, who had to invent their own equivalent. It’s a bigger savings of weight on the big 1x mountain bike cassettes than the road ones. Their architecture allows 9t top cogs too, though they themselves haven’t gone that far. The smaller front rings just keep the ratios the same. And minus a couple of chain links too.