According to the CRB index, all the rules about tiny creatures appear on p473-475. Those pages include information on how to share space and move around other creatures' space, but do not include anything about AC ability or disturbances.
This refers specifically to Chapter 9 (Playing the Game) Encounter Mode and more specifically, Size, Space and Range and movement through space of a creature.
Also, a PDF search reveals much of references to parts of objects and objects that are small, references that Familiars and Leshy Familiars are Tiny, and (perhaps the most relevant) Tiny carrying capacity and Bulk of a Tiny on p295.
However, probably the biggest indicator that there is no inherent benefit (numerical) for being of a different size is the Reduce and Enlarge spells. Shrink applies no penalty beyond restricting its Range, and Enlarge applies a damage boost and Clumsy 1 to turn it into a "buff" spell. There are no references to size bonuses, or the benefits of a creature of that size (not explained in the spell).