When Ralston finally succeeds in chipping parts of the chockstone, why do you think he collects the larger flakes?

As he progresses chipping away the softer rock underneath, he saves the large flakes. This allows Ralston a visual progress of how much he is working toward getting himself out. Although he knows the rock is slowly settling onto his arm and stays stuck, psychologically it helps Ralston feel that he is doing something to get himself out rather than doing nothing at all.