When Birch Branches are harvested, the branches are folded in and tied into tight bundles. This allows them to be easily shipped, not only because they take up much less space, but also because the branches are oriented in one direction, which minimizes the potential for damage. We untie these bundles, trim off any broken branches and fluff them out a bit giving them some more volume. When displayed, these branches have a nice dense and vertical form.
For an airier look, you simply need to get the birch branches to go back to their pre-bundling form. This is easier than it might sound - simply grasp one of the lateral (side) branches at the base, and gently pull back to about 45 degrees, which is the approximate angle the lateral branches grew at. Repeat for all of the branches, and you have what looks like a birch branch fresh off the tree.
In their opened form, the branches fill a great deal of space, and you'll need far fewer per vase. The display in the foreground of the top picture has 6-10 opened branches in it, and the vase in the background has the remainder of the 25 stem bundle.