Reduce size of plane frustrum in code

That probably didn’t make sense. Let me explain. I’m using this code gem:

public static bool IsVisibleFrom(this Renderer renderer, Camera camera)
{
	Plane[] planes = GeometryUtility.CalculateFrustumPlanes(camera);
	return GeometryUtility.TestPlanesAABB(planes, renderer.bounds);
}

which I found in the forums to determine whether something is in the camera view. Now, I want to make it so that it only detects things inside a slightly smaller frustrum, so things at the very edge of the screen aren’t included. How can I reduce the size of this frustrum while maintaining the use of this code?

My attempt would be something like this

Bounds b = new Bounds(Vector3.Zero, Vector3(xsize, ysize, 1));
return GeometryUtility.TestPlanesAABB(planes, b);