Hm...well, if reinstalling the codecs doesn't work, I'll just have to shrug and say there's a problem with the video. I'm not really sure what to tell you. Usually when you have the codec and your BSPlayer is working fine, it should be all good.
I'm a failure.
Dammit. About half of my Ayumi Hamasaki PV's are in high quality MKV format, and they play fine, and they take up the full screen when I full screen it. That is odd, indeed.