@Philreedshikes and @OldGuyot, Hi and thanks for the good advice! I've got the Osprey Aura 65 size S and loaded down, it was about 20-25lbs. I made sure that there were no clothes or anything under the belt since I had the same problem with other packs in the store, and I was afraid it would happen with this one too.
After you mentioned that the belt shouldn't be over-tightened, it occurred to me that the belt on this pack is geared to fit tightly; that is, the belt pulls back to keep it tight to you, almost as if there's hard plastic in it. If that makes sense. So even when the belt is completely unfastened, it stays snug to you and bears weight. I initially found that to be a really nice feature since it gives you so much support, but I'm curious if whatever's in the belt to make it stiff just doesn't match me or needs time to mold around me. I would find that surprising, though, since I wouldn't call my hip setup much different from other women with size small packs.
I was taught how to adjust it properly in the store, but I assume time and experience will help me learn what works for my body and what doesn't. And get used to carrying a loaded pack!
Thanks so much for the suggestions 🙂