iPhone 4 video size changes when phone is horizontal

I've had my iPhone 4 since Sept 2011, so I know it wasn't like this before. When I play a video on my phone that I recorded and rotate the phone so it is horizontal the video shrinks to just the middle of the screen. I've played these same videos countless of times before and they always filled the screen when the phone was horizontal. so how did it get like this? What did I do and how do I fix it?

Also, I have tried zooming in on the video by double tapping the screen but it just makes the screen too big and cuts off people's heads or other things.