I'm not saying that a larger package isn't more cumbersome to carry than a small one, but as you said, people have carried full sized mountain bikes up mountains, so one would think that if there were ever a task for which small folded size would take a back seat to other considerations then this would be it.

Especially considering that for the same price as a Brompton with Ti bits you could probably build a titanium S&S bike that's 7 pounds lighter, with better gearing, and with better wheels/tires for rough terrain. How could a smaller backpack outweigh all those advantages?

Even if the S&S bike isn't the best option for the OP, I would think that there are still many other bikes which would come before the Brompton. For example, a Dahon Mu SL might be a good compromise as it's only about 10 inches longer than a Brompton, but lighter than a ti brompton, with bigger wheels, and more gears. Furthermore, it's almost as cheap as an entry level brompton, so it could probably be made even lighter if the OP wanted to spend as much as he would on a ti brompton.

I'm not a hiker, so maybe I'm completely off base here, but I find that the size of a package is least important when I can strap it to my back and not worry about bumping into things (as opposed to when I need to fit it in a small compartment, like a car trunk, or a carry it in a crowded hallway, like in an office building or train). If you think that the slightly smaller folded size of the Brompton still trumps all the advantages afforded by other bikes, even when hiking, then let me ask you this: Is there any situation warranting a folding bike where you think the Brompton wouldn't be the best tool for the job?

I know, but the OP didn't specifically state that this was his intention. So I was just drawing attention to the possibility that he might not even need a folding bike.