To be clear, you can do a lot better with a better pipe, smart caching, compression, etc. But people often have horribly unrealistic estimates about how much traffic their servers can handle because they don't take bandwidth into account, and load balancers are no exception.
Of course, when you break it down by individual web request, most responses are still below 800KB, but you shouldn't load plan for the average case. And clearly even the average case is well above 12KB, especially for a CDN (which is responsible for serving the image, video, and large script content). I'm also pretty confident the page I linked already includes compression (which decreases size, but can increase time quite a bit; many people expect software load balancers to be using the absolute fastest compression available, but that's often not the case in my experience).