I just read somebody say one reason they prefer Vermiculite to Perlite is that perlite is easily crushed in shipping. I always thought that surface area was the important part of retaining water, not volume (A 2″ cube has a surface area of 24 square inches, while 8 1″ cubes has 48 square inches). Is there a size of perlite particle where it loses its usefulness?
Larger aggregate just provides more air pockets in the soil, therefore increasing drainage. The particles will have a harder time clumping together, as they don't allow each other to get close enough to form a seal, or barrier. This allows water to pass through more easily. Typically you would want to add if your soil is holding too much water, or not draining fast enough. Honestly, I prefer perlite, even if some of it gets crushed.