>The size difference is basically the number of people who dropped out.
Not hardly. There are a lot of people in other disciplines that take lower level computer science courses. Quite possibly more than are enrolled in comp sci itself.
My school separated comp sci major and non-major tracks. Very few were in the intro comp sci classes who were not intending to be majors for me. YMMV. At any rate the point is there's a very high dropout rate in computer science. Many more switch out than switch in.
Not hardly. There are a lot of people in other disciplines that take lower level computer science courses. Quite possibly more than are enrolled in comp sci itself.