What makes a game a Microgame?
The original Microgames from Metagaming had a rule booklet no larger than 32 small pages (A5 size, approximately), and a counter sheet the same dimensions, plus a map that folded to that size to fit in the plastic envelope.  Counters have to be large enough to read, so there's a limit to the number of counters that will fit on that size sheet.

Another design criterion was that they should take no more than an hour to play, ideally half an hour for experienced players.

Worth noting that within this definition fell the original Car Wars, the original Ogre, the original Melee and Wizard (that eventually evolved into GURPS), and a space combat game called Warp War, among others.