From the introduction:
A microsized creature is a creature whose’s size has been fundamentally altered by a potent effect, usually artifact-level magic or an extremely potent ritual. Microsized creatures were first introduced in Microsized Adventures, by Everyman Gaming, wherein the creature in question’s size, damage output, and ability to carry and use equipment was drastically affected by their newfound reduction in size. Furthermore, creatures that aren’t Microsized themselves (referred to as “Ordinary-Sized creatures) gain a large number of combat advantages against Microsized creatures, such as a higher Armor Class or resiliency against the shrunken opponent’s damage. In the end, mathematical changes to Microsized creatures have the ultimate effect of lowering the creature’s CR while raising the CR of Ordinary-Sized creatures. Using Microsized Adventures, it’s very possible to make even a CR 1/3 toddler a looming threat to a band of shrunken heroes.