laio, there are no viruses, AFAIK, that are referred to as micro viruses (with possible exception of biological ones that humans and other animals get).
You know a ton Mittens, but the reason there are no "micro" viruses is because righting large programs in ASM takes forever and is really hard to debug. I'm not sure if you have ever programed or read it. I have read it for cracking serials (blackhat not that I'm good at it), but it's really really hard to read. And I have tried to write ASM programs for my calculator which is like 100x less complex than a computer and that was really hard as well. Also it varies a lot based on OS. So it's not like you can write a "Windows" virus because the low level calls are still interprited by the OS. So it becomes a "Windows XP" virus. The effectiveness decreases unless you write it so that it can deal with a multitude of OS's... but the time and dedication taken to do that would be imense.

-Sam-