I had the nagging feeling we were all overlooking something. Is the rules compendium not considered a definite rules source? In it, on page 61 we find the following text on being pinned:
"When your opponent has pinned you, you are held stationary (but not helpless) for 1 round. You can't take any actions the creature pinning you doesn't allow; even speaking." The only exception to this seems to be the grapple check to escape a pin.
So, OP: Yes, you can prevent a dragon (of any size) from biting or using its breath weapon by pinning it, assuming you are already in a grapple with it. The only way to do that if the dragon is more than 1 size category larger than you is if the dragon itself successfully initiates a grapple; after that, all bets are off.