Why are sourdough crusts thicker? I'm used to the crust on sourdough bread being thick and chewy, but had always attributed that to the longer rise, and baking with steam, that one normally does with sourdough.  However, this week I made a hybrid sourdough* in my bread machine, and its crust was thicker, browner, and chewier than non-sourdough breads made in the same machine.  Since the baking cycle and atmosphere are the exact same as, say, last week's whole wheat loaf, it's not how it was baked.

So my question is: what is the physical or chemical property of sourdough starters that results in a heavier crust?

Note that while there are a number of questions on the board about sourdough crusts, all of them focus on manipulating the baking environment, and not on properties of the dough itself.

(* hybrid sourdough: some sourdough starter plus a little commercial yeast)
Here's a scientific breakdown:

Sourdough starter is the primary leavening agent in any sourdough, including your hybrid here. Technically, it's just water and flour. The flour already has all the yeasts and bacterial spores it needs to ferment, and the water activates the fermentation process via an enzyme called amylase. 

The amylase breaks down the starch in the flour into sugars, which the yeast will metabolize, producing carbon dioxide gas. Anything in the sugars that the yeast can't eat will be fermented over time by the bacteria, producing more amylase and breaking down the sugars further so that the yeast can eat it. The bacteria creates lactic acid as a byproduct of this process, giving sourdough its unique "sour" flavor.

However, the same process that breaks down the sugars in sourdough starter can also break down the proteins in the dough. This gives you weaker gluten - or, a denser, chewier bread than what you'd obtain with only baker's yeast. On top of that, your bread is a hybrid, which means there's even more yeast in it than usual and even more leavening. More leavening, more enzyme production by the bacteria, more potential for protein breakdown.