Help with problem resizing videos?

I’m having a problem resizing videos. I have a video that is 1280 x 720 and I want it to appear on the web page as 800 x 450. The largest dimension that I can get the player to resize to (even if I set it to 1280 x 720) is 720 x 412. At this size, the splash screen is 651 x 405, and the video itself is 591 x 366 (far from the 800 x 450 size that I want).

Rather than appearing with perhaps a small black letterbox or pillarbox margin at most, the video is also appearing with both of these which makes it unsightly. It was produced with a 16:9 aspect ratio (1280 x 720) and I want it to display in a 16:9 aspect ratio (800 x 450), so I don’t understand why it is being embedded with a different aspect ratio and these ugly margins.

I am using the FV Flowplayer plugin, but that doesn’t seem to be the problem, since this problem also occurs using the native WordPress video embed function, in fact when using that, the video appears even smaller. I have also deactivated all the plugins except Flowplayer and the problem still occurs.

AND, this problem DID NOT occur when I embedded two other animations with Flowplayer that are also 1280 x 720, but do appear on the web page as 800 x 450 with only slight letter OR pillar box margins, as I want them to. The only difference is that the video is much longer than the animations so I had to upload it to my host’s server via FTP, however I did transfer it to the WordPress media library where the dimensions are correctly indicated as 1280 x 720.

Any ideas how to fix this problem?