Those mile-high apple pies take a lot of apples and, if you chop them too small, they’ll break down and settle, leaving a gap. Try using firm apples such as Granny Smiths that hold up during baking. If you do want to use softer apples that will shrink, like Red or Golden Delicious, or McIntosh, slice them thicker and they shouldn’t settle as much. Also, a butter crust shrinks more than a shortening crust, so it will hug the filling as it cooks. Finally, you need vents−let the steam escape and the crust won’t puff up, leaving a gap.