There are some newer heli's coming out now that defy (what I thought) was conventionally the difference. Aside from just slightly smaller size, micro's use a separate motor to power the tail, where mini's use either a belt or shaft to draw power for the tail from the main motor. The Axe, in my opinion, looks beefier because of the it's full frame. To me, it's a lot more attractive than the Blade CP and Honeybee King and stuff that look like a bunch of sticks with a rotor on top.
Anymore, from reading specs and stuff of various heli's, it seems that the only distinguishing characteristic that all the 'micros' share is a main rotor diameter of 20.75" or less.