<i>Much of the die is unused, especially the large metal area in the middle.</i><p>I don't think the word "unused" is appropriate.<p>1) Probably they wanted to physically separate the comparators, for better noise isolation between them.<p>2) the large metal area is connected to V+. By using such a large amount of metal they get slightly lower voltage drop in their power distribution.<p>3) But here's what I think is the main reason. The metal is being used as a power decoupling capacitor. I can't be sure of what's underneath, but most likely the area is connected to V-. Voila! You couldn't possibly get a decoupling cap any closer to where it's needed than that.