The other issue is ‘heat quality’ (or temperature deltas). It’s easier to recover a given amount of heat from very hot sources than from only luke warm sources, and smaller heat stores will have a very pronounced difference very quickly, resulting in control issues.

Basically, if you only have a small amount of sand, it’s harder to keep it warm for longer (more surface area for the mass, so more insulation required), and harder to deal with effectively (fan needs to blow much harder when it’s only warm, much less when it’s hot), etc.