@adamhcaldwell
"When you make a custom orthotic is it to fill the volume under the arch? Or just control how far the arch is collapsing?"
Not that simple. You have to look at the quality/quantity and relationship of the rearfoot/midfoot/forefoot. @BraceMaker's image of the main ankle joints are spot on- lots of variability for angles/axes and compensations from one area to the next.
The rearfoot needs to be addressed first as it is the area that has first contact. There is a subtle lateral motion in the heel that allows our foot to pronate and create appropriate tibial rotation and load up to the hip. A 2 degree adjustment there could have a profound effect upon the foot as a whole and may give you the control you need without needing as much of an arch. Yes, the fill of the arch is there to "catch" the foot and ensure stability/dispersion of forces.
A midfoot or arch support is dependent on the natural alignment and control of the foot itself. It is quite possible that when you restricted your midfoot/forefoot and required your ankle to complete more of the 3D movement. A lower profile device or with a more intrinsic build would be best to lessen volume.
I ski in San Diego, though make it back to Massachusetts w @Pat M during the summer at least once a year. I'm interested in seeing if an orthotic will improve my skiing this year, I have used the Reflex/R-style for years.