I am skinny and sore hips have always been an issue with loads above 20lbs. After a week or so, it goes away. This has been the case with external frames (several Keltys) and several brands of internal frames (Mountainsmith, Gregory, ULA, Deuter, an REI Venturi, and a GG Mariposa Plus.
A couple years ago I got a McHale LBP 36, which has was Dan calls a SARC hip belt. I don't know why, but I have NEVER had any soreness or even a hint of discomfort with this pack. Often my first day on a desert trip can include up to 17 lbs of water.
This year I got a Bump pack from McHale. He offers some different options for belts (to save weight). I did not even consider them at all, and had the pack built with a SARC hip belt. Again no soreness in the hips with this pack either. Also, both packs NEVER leave my shoulders sore either... this is a function of a perfectly fitted pack, outstanding integrated design of the frame, structure, shoulder straps, hip belt, and coaching from Dan on how to properly adjust a pack.
It is not unusual for me to do 20+ miles per day with either pack.
This is one of the reasons why I often give McHale packs a testimonial on BPL, which seems to irritate some folks. But they work better than anything I have ever used and I have probably owned around 20 packs over the past 40 plus years. Before the McHales my base weight was usually SUL, but I am now thrilled to have an 8-10 lb base weight using on of Dan's packs. They make that much difference!!