Lovely question. On “hiding flaws”, it cuts both ways, I think.
On a small, there are fewer quilting diamonds across the front, so each one is larger and any irregularity in one diamond stands out more. The CC turnlock also looks proportionally bigger, so its finish matters more.
On a medium, there are more diamonds to line up across the flap and the back pocket. Lining them up is harder, so misalignment shows across the panel rather than in one place.
The Flap 25 tells thread goes through the medium’s specific details.