Well if there's not enough white matter, then the conduction of nerve impulses between different brain areas will be impeded and as a result signals may take longer or fail to reach the places they need to go. In the case of stuttering, this could be like the basal ganglia part of the brain trying to send speech motor plans to the brocas area of the brain and since the signals aren't reaching or taking a long time to get from one place to another; we have speech blocks where we're unable to execute a speech sound or it takes a some time until we're able to say that sound.