How is it that brioche rises so high? I just made this brioche and I'm very pleased with it.




I'm puzzled though. Where did all of this lift come from? The dough shaped in the pan for the final proof was tiny. 

The recipe calls for a sponge made with 1/4 tsp yeast, 1 TBS sugar, 1 egg, 1/2 cup flour and 2 TBS of water. That can ferment for up to a day, but I only allowed a couple of hours. It was pretty unremarkable. 

Then I sprinkled over the sponge just over 1 cup of flour, 2 TBS sugar, 1 1/4 tsp yeast and 1/2 tsp salt. That sat for an hour, then I kneaded it (for no longer than normal bread), adding 1/4 lb butter and 2 cold eggs. That rose until doubled, then I punched it down and left it in the refrigerator overnight. The next day I formed the loaf in a normal loaf pan.

This is where I got nervous. 

I didn't think there was any way that this tiny mass of dough was even going to rise to the top of the pan. It looked like a Twinkie!

A 2 hour proof did see the dough rise just to the top of the pan. 35 minutes at 350F caused it to get just massive (relative to normal bread).

How? The eggs aren't whipped, it doesn't call for a tremendous amount of sugar and it takes just over 1/2 the amount of yeast that would be found in typical sandwich bread. How did such a tiny amount of dough turn into such a big loaf?
http://www.weekendbakery.com/posts/bread-baking-tips-making-the-most-of-your-oven/
This post suggests to me that the dough was moister on the outside as it was baking, which probably allowed the bread to spring more than a regular bread dough.  I am guessing that this is a function of the eggs and butter that are in brioche dough.  In reading about butter in Ruhlman's Twenty, some of this may be the butter coating the flour, and preventing the stronger gluten strands that might have kept the spring from happening.