I watched a bit of the Cotswold video, the saleslady suggested a 60-65L because her customer was planning on hiking the Pennine Way. I carry a 48L, more than enough for me and I've still got the room for cooking equipment if I ever chose to carry such stuff. I could imagine a 55L to be my absolute maximum but not more.
Anyway back to the problem which I have to confess has never happened to any of my clothing. From the photo, it looks as the nylon material has bunched up from, as already suggested by others, the possibility that the waist strap may have been tightened over the squishy tummy area instead of securing it just below the Iliac Crest of the hipbones.
When hiking I always secure my waistband first, then my shoulder straps and then the horizontal strap. I feel most comfortable if I'm carrying the rucksack weight on my hips, the shoulder straps I just pull them tight enough to secure it comfortably to my back and to avoid any oscillating movement on my shoulder bones.