How is it that brioche rises so high?
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.