“A significant portion of the overall size of a game is comprised of texture data,” Jason Ronald, Director of Program Management, explained on Xbox Wire.
“As Xbox Series S was designed with a performance target of 1440p at 60 FPS with support for up to 120 FPS, many games will not require their highest level of 4K textures resulting in smaller game sizes overall, often up to 30% smaller on average.”
The difference in install sizes is basically the linchpin of Xbox’s Smart Delivery service, which only downloads the elements of a game that are needed, depending on the console you’re playing on. The Xbox Series S, in the example above, doesn’t output in 4K so won’t download 4K textures. A similar service is currently employed between Xbox One consoles.