The amount of water you add to the dough affects how open the crumb is in the final result
The type of flour used in your mixture will make a massive difference to the end result of your bread
If you still choose to use whole wheat flour, try sifting some or all of the flour to get rid of part of the bran.
Soaking whole grain flour overnight before adding it to the final dough mixture will allow the bran in the wheat to soften and become more flexible.
Steam retention during baking is extremely important when it comes to getting a good rise in sourdough bread.