I'm so glad you mentioned this as I've been meaning to for a few weeks now. I know what you mean and I have exactly the same problem with all my bib shorts : Castelli, Sportful, North Wave. All shorts were one away from the top of the range of respective brands. I've engaged the suppliers and followed their care guidelines exactly - gentle wash, lowest spin cycle, don't hang to dry, dry away from sunlight, etc. I even take them off before doing any stretching or rolling on the foam roller. But still keeps happening to all my bib shorts after only about 3-6 months old and worn / washed no more than once a week. On the back, above the pad the fabric seems to stretch and thin out to the point where it's see through. I even thought it's maybe from keeping stuff in the back pocket that rubs against the fabric but my shirt pockets are no where near where it's thinning but I still keep nothing in that pocket anyway. I'd love to know why this happens to some of us ? My current theory is that I might be in the tall side of my size and maybe the fabric in this area is being stretched but I cannot take a larger size.