Physics/chemistry behind the observation

McGee on Food and Cooking: An Encyclopedia of Kitchen Science, History and Culture will answer most questions on what the chemistry is behind most cooking processes, without being too academic.

I always thought, and one of my molecular cooking books indicates that, smaller batches kept the heat higher (less temperature shock), and the scraping released starch. The article Key success factor for tasty and creamy risotto on ideas-smart.com indicates something slightly different:

Note that the instructions normally suggest that each batch of the stock needs to be absorbed by rice. In reality, something else is happening. Most of the water actually evaporates. The remaining “mash” gets warmer than boiling point. This speeds up the chemical processes, which create the intense taste of the risotto. These chemical processes are essentially the same as when browning meat.

Other reason for pouring the stock in small batches and stirring constantly is to let loose starch from the rice kernels. Starch makes the mixture creamy. So the tedious process of adding stock in small batches and stirring constantly is the key to having tasty and creamy risotto. Only the reason for doing this, which most recipes give, is often misleading.

This matches personal experience and some (other) of my cook books.