In this example, the size of the grass clumps are too large for the scale of the scene. You can use the Size attribute to scale the geometry, and then increase the number of grass clumps by setting Density.
Increasing the Size value decreases the scale of the geometry.
In the scene, all the grass clumps are the same size, which makes them appear unnatural. An easy way to vary their size is to rescale the guides.
When you preview the Description, notice the variation in the size of the clumps. The size of each primitive is now an interpolation of the different sized guides.
Later in the example, you use expressions to better randomize the size and orientation of primitives.