Ask CakeSpy: Shrinking Cupcakes?

Dear CakeSpy,

I have a technical question for you.  I tried making cupcakes from a Billy's Bakery recipe I found online and the cupcakes separated from the liners. Any idea why this happens?

-Shrinking in NY

P.S. Here's the recipe:

Billy's Bakery Cupcakes

• 1 3/4 cups cake flour, not self-rising
• 1 1/4 cups unbleached all-purpose flour
• 2 cups sugar
• 1 tablespoon baking powder
• 3/4 teaspoon salt
• 1 cup (2 sticks) unsalted butter, cut into 1-inch cubes
• 4 large eggs
• 1 cup whole milk
• 1 teaspoon pure vanilla extract

Procedure

1. Preheat oven to 325 degrees. Line cupcake pans with paper liners; set aside. In the bowl of an electric mixer fitted with the paddle attachment, combine flours, sugar, baking powder, and salt; mix on low speed until combined. Add butter, mixing until just coated with flour.
2. In a large glass measuring cup, whisk together eggs, milk, and vanilla. With mixer on medium speed, add wet ingredients in 3 parts, scraping down sides of bowl before each addition; beat until ingredients are incorporated but do not overbeat.
3. Divide batter evenly among liners, filling about 2/3 full. Bake, rotating pan halfway through, until a cake tester inserted in the center comes out clean, 17 to 20 minutes.
4. Transfer to a wire rack to cool completely. Repeat process with remaining batter. Once cupcakes have cooled, use a small offset spatula to frost tops of each cupcake. Decorate with sprinkles, if desired. Serve at room temperature.

Dear Shrinking,

It's true: not all cups runneth over. But I must confess, and I promise I am not saying this in a "I'm such a great baker" sort of way--I've never actually had it happen, so I wasn't quite sure what might be the cause. Happily, I was able to consult the world's most reliable information source--Twitter, natch!--about the subject, and received many ideas as to what might be the cause.

One thing which several respondents mentioned was humidity as a shrinking factor: it being that you were baking in the summer, in NY, it could be as simple as blaming it on the weather!

Another widely accepted response was that the shrinkage could be due to over-greasing cupcake papers, which can cause separation (even more so depending on the type of paper used).

However I also received some other interesting responses:

As Bakelab, a Los Angeles-based retail bakery and baking school informed, this can often be because of a high amount of sugar in the recipe--and while 2 cups is certainly normal in cupcake recipes, it is a pretty substantial amount. A friend once told me that the sugar in recipes can generally be reduced by up to 1/4 cup without sacrificing the structure of a baked good--she might have been lying to me, but I'm just passing on what I've been told.

Ryan's Baking Blog also pointed me toward this posting on the subject, which notes that often, shrinkage can occur when the cakes are overbaked, but that other culprits may be too little mixture in the pan, over-greased trays, over-beating your batter, or too much liquid.

Unfortunately, all of this doesn't add up to one easy explanation, but it may give you some ideas for next time! Of course, if the cupcakes still tasted good, I say shrink away, since I'm gonna take the wrapper off anyway.

If anyone else has ideas as to what might have caused this to occur, please leave a comment!