Black bars in video using Windows Media Player

[MagicBoy99]

What determines if a video is displayed in full screen or if there are black bars on the sides of top and bottom? Some of my MP4 videos show bars while others don't.

 

[johnebadbak]

widescreen format has black bars some are 16.9 and show full screen

on vimeo site "the changing shape of cinema" interesting view