This is the same cookie batter, chilled for the same amount of time, baked for the same amount of time. The one on the left was baked on a cookie sheet lined with a silicone mat, and the one on the right was baked on a cookie sheet lined with a parchment sheet. I have absolutely no scientific evidence to back this up, but in my experience, cookies always spread more and fall flatter when they're baked on silicone. Those baked on parchment always seem to maintain their thickness much more, and have a much better texture. Maybe silicone is too non-stick and the cookie dough spreads out too easily, making it flatter? I have no idea why. But every time I give silicone another chance, I see why I stop using it and switch to parchment sheets. I hate it when I ruin good cookie dough!
Oh well. I ruined a few cookies, but I learned that I should only use my silpats when I'm baking items that I don't need to worry about spreading. For everything else, I'll stick with my perfect parchment.