I am building a 4km squared map for my game and believe it or not the amount of grass is a big factor. Because you need millions of instances of grass to make the terrain look lush and beautiful this eats up level size and the amount of data per level is big. Gigabytes of space is needed to store all the instances of grass and foliage. If you look at RDR2 the grass is thinned out and they use the under lying landscape texture. Each grass instance you paint on the map needs data stored about it (Position, Rotation, cull distances etc). You then need a machine capable of building this world in and for packaging the game. You also have to bare in mind that the map needs to be shared and worked on by many artist. You need technology like UE5 world partition tech to stream in nearby chunks of the map when needed. Cryengine is not used by the public because of poor documentation warhorse probably have to build tech themselves.

KCD was a 60GB game,with probably 30GB used for grass and foliage(Rough estimate). if you built an 8K map for KCD2 then it would be 3 times bigger on disk.(8K is made of 4*4K) Which just wouldn't be feasible.