Downloads "complete" prematurely

When downloading this Youtube video here at http://www.youtube.com/watch?v=Odmvx-T8uJc at 1080p 60fps with the length being 0:48:56 I am told that what I am about to download has a size of 1.5 GB - which is fine. However what is not so fine is that the download prematurely "completes" with only 84.3 MB being downloaded. It has now happened twice? Why is that?