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.)