Why does bandwidth (MB/s) decrease over larger sizes? Is it possible caches play a larger factor during smaller memcpy, and you see the real CPU<->RAM bandwidth

Why does bandwidth (MB/s) decrease over larger sizes? Is it possible caches play a larger factor during smaller memcpy, and you see the real CPU<->RAM bandwidth when you’re touching larger areas of memory? EDIT: never mind, your comment seems to indicate that to be the case