A creature of this size takes up less than a 5-foot-by-5-foot space (1 square on the grid), and multiple Tiny creatures can occupy the same square on the grid.
Kind
Size trait
A creature of this size takes up less than a 5-foot-by-5-foot space (1 square on the grid), and multiple Tiny creatures can occupy the same square on the grid. At least four Tiny creatures can occupy the same square, and even more can occupy the same square, at the GM's discretion. They can also occupy the same space as larger creatures, and if their reach is 0 feet, they must do so in order to attack. Other Traits After any rarity traits and the size, each creature's trait line lists all other traits the creature has in alphabetical order. The following is a list of those traits found in the book.
Other size
Tiny is one of 6: Gargantuan, Huge, Large, Medium, Small.
Source: Pathfinder Monster Core, p. 361
Bring Tiny into a Pathfinder 2e game with The DM Screen.