Ya, I hear what you're saying. Assuming your figure of 3 square inches is right, I'm putting 100 lbs on each inch - at least at the beginnings of a cross-country marathon. My biggest problem wasn't the sit bones, as much as it was chafing - I would actually bleed at times and could hardly walk at the end of a day. My second summer of riding I solved my problem pragmatically (I don't care what it looks like, as long as it works) - I simply added a 3" very solid foam pad to the sit bones area (another good use for duct tape!), then covered it with a stuff sack to keep it from soaking up rain water. Not only did my chafing stop, it was downright comfortable -- in fact in the morning when I get on for another long day, I've caught myself saying, "Ahhh, this actually feels good!"
So good in fact that I've been able to put on 12,000 kms (solo) in the past 3 summers -- not too shabby for a big ole fat guy!