While I love the notion that Lucuna is a tooth fairy, I'm sticking with my gut feeling that Lacuna was force fed by Ace, as Ace was trying to duplicate Harry's guard.  Lacuna thinks the pizza made her bigger, which from Toot, we understand is considered a bad thing.

Much like being fat.

I don't think its the pizza making Toot bigger. I think its the experiences. He's taking on bigger, more serious roles. The more responsibility, courage, and serious triumphs he takes, I think he'll become more like a Sidhe than a pixie. I think Lacuna is similar.  I think its a metaphor for leaving childhood (little pixies are very innocent (peter pan like) and fickle) and growing into adulthood.