Greetings Iaio,

Your question would depend on your definition of "micro"

As far as I am aware there is no malware written in microcode, simply because the authors cannot access the tools to write it and would have no means of delivering it. Also, given that microcode is specific to the internal functionality of a computer, it would be difficult to get it to spread.

If you are just referring to size, some of the earlier boot sector viruses written in assembly language were very small, less than 2k

"Ordinary" malware is written in a variety of higher level languages, and is quite "bloated" by comparison.

Try looking at the various definition pages of the AV suppliers' websites and check out a few boot sector viruses, file infectors, worms and trojans. You will soon see what I mean.

Cheers