what do they signify?
A participation system - the bigger the pie the more participation and they keep increasing.
If you roll your cursor over the pie, a box should pop up explaining how the pies work. Please let us know if you're not seeing the rollover pop-up.
Someone on another question noted that they didn't like the pies, as they might not seem welcoming. But my personal experience is that the FOOD52 staff is pretty liberal with what's considered participation. For example, I don't store recipes here (rarely make the same thing twice) and have never entered a contest, but they've fill my pie in pretty well. Thanks, guys.