video only takes a quarter of the screen

[effemmeffe]

don't know how to take screenshots.
I checked to have disable_overscan in the configuration, I searched on the forum, but nothing worked.
Then I changed the GUI resolution and set it to the same value of the video I wanted to watch: 50 fps and suddendly everything went well.
So in the end the problem is this: if I set any GUI resolution but 50 fps and I try to watch a movie at 50 fps the resolution changes at the beginning of the movie and the screen will resize to a quarter of my TV screen.