I forget another explanation for this. It was quite some time ago on the forums.

I generally try different players. My favourite or VLC & MPlayer: CODECs list and alternate players for Movies

Oh ... BTW if you're previewing a file whilst it's downlding & this happens ... it's because files don't necessarily download sequentially. The 1st & last & some middle parts might downld 1st & the rest of the downlding process is filling in the holes/gaps. That's why sometimes you start downlding a video file & it immediately shows its complete size on the computer despite it might only be a few % downlded. The computer system sees the start & finish & thinks ok yes it starts at 0.00 & finished as x.xx with the info supplied it runs for x amount of time, & I think also relates to file size in a similar fashion.