You video have a size of :
640*360 --> ratio : 16/9
The player is set with a size of
760*320 --> ratio : 21.375/9
So it' s not the same size and worst it' snot the same ratio.
So two way to adjust this set the same size for the player (at least same ratio 16/9) and/or play with the "Stretching” parameters :
• Field “Stretching”
How to resize the poster and video to fit the display. Can be none (keep original dimensions), exactfit (stretch is proportionally), uniform (stretch proportionally; black borders) or fill (stretch proportionally; parts cut off). Defaults to uniform.