The material below is Open Game Content from a 3PP Source
Puff Up (Ex): An eidolon can fill itself with gas like a puffer fish, taking on a greater size. As a move action the eidolon can become one size category larger. Its weight, reach, carrying capacity, and ability scores do not change, since only its central body grows in size (becoming rounder) and its extra volume is all air. However, the eidolon is considered one size larger for purposes of the space it takes up, modifiers to maneuvers, and other size modifiers. An eidolon can maintain this increased size for 1 minute per point of Constitution score, then must deflate (a swift action). It also deflates if stunned, knocked unconscious, or killed.
Once deflated, an eidolon cannot puff up again until a number of minutes have passed equal to the time it spent inflated.