Fine, Diminutive, And Tiny
These creatures take up less than 1 square of space. This means that more than one such creature can fit into a single square. A Tiny creature typically occupies a space only 2-1/2 feet across, so four can fit into a single square. Up to 25 Diminutive creatures or 100 Fine creatures can fit into a single square.
Source
Core Rulebook pg. 255