The off-axis mirror has been sized to match the diameter of the light cone of the guide star delivered by the telescope. They don't produce brighter guide stars. They need the larger prism because their design places the prism in a region of the light beam where the diameter of the light cone is much larger. The problem with a large light cone is that there is a limit to how much of it can be captured without shadowing the camera image. In this case, even though a large prism is used, only a fraction of the light is captured.
Image Shadowing A common concern with guiders is image shadowing.
This is caused by two competing requirements. The off-axis prism or mirror must be inserted into the periphery of the light beam supplied by the telescope in order to see the guide star. The deeper the insertion, the more guide stars are available and the brighter they will be due to the natural vignetting characteristics of the telescope. However, if the insertion is too deep, it will partially block the light for the target image. This is called image shadowing.