Brightness and contrast in HD or SD video content - and small buttons

Hi there...

just watching a movie on my XBMC and wondering about a bit too dark picture. So I went to the menu in the movie and rise the brightness a little bit and everything was fine. But...

I only have this feature in SD (DVD) content and not if I play HD video content. In HD-content I don have the possibility to change brighness or contrast. It's not grey-out...it's simply not there!

Is this correct and if "yes", why?

And I noticed, that I don't have the option to set-up brightness or contrast in the XBMC settings. Do I have to change this in XBMCbuntu?

If yes...I got another problem. To get the best picture I will have to plug my XBMC to my beamer. If I do this, every button is so small, that it is impossible to read something. If I plug the XMBC to my normal PC-monitor, everything is fine and all buttons and letters are in normal size. How am I able to get this fixed.

Thanks for the answers

Greetings David