OK. Let me explain. H264 stream consists of P/B frames and I frames (keyframes). Typically for 1080p P/B frame is about 20KiB, keyframe can be 300 KiB.
Let's assume that keyframe is going every second and you have 5 fps.
So bandwidth usage will be (KiB) 300, 20, 20, 20, 20, 20, 300, 20, 20, 20...