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 suggestive to the browser user agent. That said, preload behaves differently on different browsers because the preload attribute is only suggestive

Also, preload does not work on mobile devices, however here is an article that shows browser compatibility for preload. One other thing to keep in mind is that the autoplay attribute will automatically disable preload.