Quote Originally Posted by IGN
In an interview with IGN, Xbox director of program management Jason Ronald confirmed that game install sizes will be approximately 30% smaller on the Series S than on the Series X. This is due to the reduced resolution texture packages needed for games on the Series S, which will be targeting 1440p resolution instead of 4K on the Series X.

“With a performance target of 1440p at 60 fps, our expectation is that developers will not ship their highest level mipmaps to Xbox Series S, which will reduce the size of the games,” Ronald said. “Ultimately the controls in the developer's hands. We've had this technology for a while that allows developers to intelligently choose which assets to install on which device they're playing on. So the flexibility is in the developers’ hands to make sure the right assets are there.”
Xbox Series S Game Sizes Will Be Smaller Than on Series X - IGN