Well... I don't know if I'm saying crap or if I'm right... but it's good to think...
MBR is stored in your HD, ok... so... if your hd has a hardware limitation of the MBR size (I really don't know if this exists)... I'm afraid there's nothing you could do... if it is because of security reasons or any other thing... I don't know too (not helping very much, am I?), what you could do is to enter your HD manufacturer site and search for advanced information on the model you have, anyway, I didn't help you telling this :P