I've been considering for a while that big dudes should just be plain harder to kill. To a certain extent, Constitution covers this, you know? I dunno, maybe I'm kind of barking up the wrong tree here, but, we'll see. Ok, so a big dude, like a huge dude, simply has more mass to cut up and hack through to get at his insides. A lot of things can represent this: the Toughness feat and other feats like Endurance and Diehard, and an eighteen in Constitution is considered to be a superhuman level of stamina.
So I'm looking at the rules for the Construct type of creature, and they don't have Constitution scores. However, because they are so tough, they do have bonus hit points based on size starting at ten for being small and capping at eighty for being colossal. Now, I am not suggesting every seven foot tall Child of Volung should have twenty bonus hit points to his name or something like that.
What I am considering is that creatures of large size gain a one time bonus to hit points equal to their Constitution modifier. Perhaps something along the lines of for each size category larger than medium, living creatures gain bonus hit points equal to their Constitution modifier. Designed in this way, the feat is mostly irrelevant for normal characters. Which is fair, as they are normal. Larger characters, like some Soulless, Rankethlek, or Fell Humans, unnatural creatures with superhuman sizes and abilities, would make the most use of this rule, it would also offset that fact that larger sizes are something of a penalty. Except when you are wielding a massive two handed sword to hit a guy from twenty feet away, heh.
While I'm thinking of it, would it be appropriate to create a feat or something that bases your hit points or Fortitude saves off of Strength, rather than Constitution? I vaguely recall a series like that in some of the Complete books from 3.5. I think there were some that allowed you to kind of switch up your saves the way 4th Edition allows you to base your defenses on the higher of two stats.
I dunno, just a few thoughts about hit points I guess.