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.