Micro games, which pretty much always refers to the board or card games, have relatively few pieces, take a little storage space, have a relatively small footprint and A relatively short playing time. In other words, games that are easy to transport and play on the fly.
For instance, I have carried around Pico 2 for years because it takes up almost no space, takes no time to teach and can be played at the drop of a hat.
On the other hand, micro RPG simply means that it is a very short rulebook. Often, only a couple pages. However, they usually take up about as much space as any other role playing game, unless you are using a mat and miniatures. In which case, you're definitely not in the market for a micro RPG, anyway.
While micro RPG's tend to be for one shots, as opposed to campaigns, there are plenty of one rule books that are designed for one shot games. Fiasco, just to name one that does a really good job at it. Frankly, I could go on for a while.
And, unlike micro games where simple rules make it easier to learn and play the game, sometimes too simple, the sandbox nature of role-playing games makes a super short rulebook actually an impediment. Too much ambiguity and not enough explanation.
When I look at a micro game, I ask myself if it would be any good to play and if I am likely to ever get it on the table. When I look at a micro RPG, I ask myself if it's fun to read.