Because the version they put on Youtube is probably not the exact version they received from the ISS. The article repeatedly described the video as HD, and the Youtube version isn't. And if you aren't willing to take NASA at their word, it's not that hard to imagine how multiple versions of a video end up getting passed around an organization, and the one that gets uploaded is compressed for whatever reason.

It's certainly possible that it is of similar size. For it to be meaningful, I would want it to be overwhelmingly likely that it was the original.