That last remark was a bit flippant, but have been thinking about this a lot (several years since I did my dissertation on it).

Try this as a thought experiment:

Imagine a huge solid block of a material, does not matter what it is.
Put a thermometer in the middle and imagine what the temperature will be.
No slowing remove the material in the middle and replace with air, now imagine what the temperature will be.
Keep doing this until there is just a shell. What will the temperature be?

Perceived wisdom has it that the more mass there is, the more stable the internal temperature will be for varying external temperature forcings. This is backed up with evidence and is also what GSHP rely on, but not ASHP.

The questions being asked here are:
What is the best material to use
What is the best location for that material
What is the best amount of that material.
Does it make any practical difference