Smaller soundboards stay more resonant because Schroeder frequency scales inversely with size

Evidence

The Schroeder frequency is a function of the size of the board. If everything is scaled up or down, including the board thickness, a good estimate by the law of similarity (which is nearly exact for air, but has restrictions for solid materials) is that the Schroeder frequency should scale as the inverse of the linear dimension of the board. A small board can be more resonant (in the sense of large, isolated resonance peaks in the audio range), as shown in figure 12. — Eric J. Heller, Why You Hear What You Hear- An…, 1177

Categorization

• CANDY: Small instruments sound ‘boxier’ because they stay in isolated-resonance territory longer - a fun, shareable acoustic fact.