The Glow Urchin originates from the film Monsters University. In the film, they are used as obstacles in the first event of the Scare Games. If they touch any part of a monster's body, the exposed part swells up. In Disney Infinity, it is a tool. It is unlocked by finding all 20 Monsters University pennants, hidden around the campus. When it hits someone, the target bloats either at the waist or head (depending on the character's gender and body type) and deals minor damage.
Mrs. Incredible is currently the only known playable female character in the entire Infinity series that has her waist bloat in-game via the Urchins (not counting the WALL-E Hoverchair a very similar item that also causes bloating, but always at the waist), every playable female character (since Violet onwards) all receive head expansion instead.
It's possible to remove the bloat effect via using another effect from a character in-game or another effect from an enemy (Invisibility, Glitching, etc.)
When a character is under the effects of the Glow Urchin their voice will deepen a bit similar to the Goo Grower's effect (only not as deep), this was removed starting in 3.0 (likely due to Darth Vader's playability).
Starting in 3.0 almost all playable male characters introduced in 3.0 receive waist expansion regardless of body type (with the sole exception being Mickey Mouse as he's the only known playable male character in 3.0 who has head expansion, likely because he shares almost the same model as Sorcerer's Apprentice Mickey who also has head expansion when struck by the Urchins.
Hulkbuster is the biggest character in-game to receive waist expansion via the Urchins (due to him being a variant of regular Iron Man who also has waist expansion), likewise the smallest characters to receive waist expansion are Anger and Olaf.