Yes, it has changed a lot since then and also put on some heavy weight in kernel and userland. The goal of Minix 3 is to become a highly reliable, flexible, and secure OS which can be used as a product in a serious way. Visit http://en.wikipedia.org/wiki/MINIX_3 for a comprehensive information.
You are right to some extent. Yet, there has been many improvements in the kernel since 1988, especially after Minix 3 was announced. And yes, the code base has significantly grown in all of drivers, user-space servers and userland.
