Why Do Bigger Things Take Longer to Cook?

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Can you give me a scientific explanation of why the larger the cookie the longer it takes to bake (the dough, oven temperature, pan, and shape of the cookie all being constant)?

The middle is further from the heat. It takes longer for heat to thoroughly penetrate a larger object than a smaller object of the same material.

If that's not scientific enough, you might want to check out The Science of Cooking (Canada, UK), by Peter Barham. In the chapter, Heating and Eating — Physical Gastronomy, he gives a detailed description of how heat penetrates and cooks food, and accompanies it with graphs and equations on "thermal diffusivity and the heat transfer equation," which we, alas, are much too simple to grasp.

(The book actually offers fascinating and understandable glimpses of many of the scientific processes that enter into cooking, as well as down-to-earth recipes that demonstrate these principles. We have simply trained our eyes to skip over the occasional scientific formula or mathematical equation — surely much to our loss.)