Why does preload not load my entire media file?
According to w3.org, preload has one of three methods ("none", "auto" and "metadata"). Each setting is only "suggestive" to the browser user agent. Read more here: https://www.w3.org/TR/html5/embedded-content-0.html#attr-media-preload. That said, preload behaves differently on different browsers because the preload attribute is only "suggestive".
Also, preload does not work on mobile devices according to https://www.aerserv.com/why-does-video-autoplay-on-mobile-devices-not-work/.
One other thing to keep in mind is that the "autoplay" attribute will automatically disable "preload" (see this site: http://www.w3schools.com/tags/att_video_preload.asp).