Reasoning 1: On a general base, you have more surface area exposed so more air molecules (if you're not having a meal in outer space) come in contact with the food, absorbing heat energy, cooling it down.
Ok, so this is only one reason if your food is dehydrated 'to the core'.
Reasoning 1 + 2: Most foodstuff contain moisture. This moisture evaporates on absorbing heat energy from surroundings. This causes a temperature decrease of the food surface.
Looking upon the basic factors that alter evaporation rate, more surface area exposure causes a faster evaporation rate, hence, faster cooling rate.
