Well, I mentioned it in regards to the Gorilla. But establishing a solid hip belt to stay (internal frame) connection is very important to hauling heavy loads. I sort of dislike to say much about this since I am a trail ambassador for Gossamer Gear, but this was brought up with them more than a year ago, so I feel free to talk about it. I used two different sizes of Gorilla to see if this would be present regardless of the loading on the shoulders: a large and a medium. (Though this was not the primary reason for the different sized pack.) In analysis, I could determine two basic factors for the low weight limit on the Gorilla (and the Mariposa, it has the same stay and multiple hip-belt attachments.)
1) I have found that velcro will creep after a while under constant loading with a pack. This is likely due to simple walking. Statically speaking, it is well over done. But dynamically, it will stretch and creep. Over a few hours of 25-30 pound loads with a Gorilla, this can be about 1/2 – 1" (or about 1.5-2.5cm.) This is enough to put extra pressure on your shoulders and allows the frame to drop not quite fitting your back, anymore. The pack starts to feel uncomfortable with heavy, near limit loads. The impact moment of each step is a lot more than what the velcro is designed for. It stretches and rebounds at each step. Sometimes one of the hook&loop's will make a new connection slightly higher, hence the creep.
2) A solid mount between the belt and stay means there will not be any creep (well, you might get some fabric stretching at 30-40 pounds.) It stays where you adjust it.
3) The too-thick padding on the hip belt also contributes to this. It is just too thick. It feels very nice when you put it on. But, the stresses of walking cause it to compress and become overly large. Soo, it slips down. Unless you tighten the snot out of the hip belt(too tight for comfort,) it doesn't support loads while walking very well. Well, at least for me with my…ah, hum…more rotund figure. Women, with their somewhat larger hip structure probably don't have this problem.
THE TRANSFER OF ALL THE WEIGHT OF A PACK WILL BE THROUGH THE HIPS TO THE LEGS. Carrying weight on your shoulders is only for balance. I almost never need more padding on my shoulders other than a thin 1/8" or 3/16" piece of light foam. Of course, if you use a hiking staff or two, you can short circuit some weight, but your arms were not meant for this. Walking auto-magically carries your weight on your legs. Those packs that carry 50 or more pounds of weight always have a solid hip belt connection to the pack. Often, there are adjustments on the hip-belt to make it comfortable for the hiker. So, looking at large capacity internally framed (Gregory for example) and externally framed packs (Kelty for example) this makes sense, too.
I described the process of the modification in the other post: http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=105637&skip_to_post=892398#892398
