Saponins as the surfactant
Green tea contains natural surfactants called saponins. They lower the surface tension of water around incorporated air, which lets fine bubbles form and hold their shape. Extraction of these saponins peaks at around 70 degrees Celsius, which is one reason 70 to 75 degree water tends to foam best.
Particle size matters
Stone milled matcha has particles around 5 to 10 microns. Finer particles disperse more evenly, attach more readily to bubble walls, and stabilize the foam. Coarser, machine ground powder tends to settle out and produces a thinner, shorter lived crema even with identical technique.
What the chasen does
The many tines of a chasen subdivide air bubbles as the tea passes through them. Each pass through the cluster turns a few large bubbles into many smaller ones, which is how a coarse froth refines into a tight microfoam. A fork or spoon cannot replicate this action, which is why chasen made matcha looks visibly different from shaker bottle matcha.
Why foam is not a grade signal
Foam volume tracks saponin content as much as quality. Some excellent ceremonial matchas foam less than mid tier matchas because their cultivar profile differs. A tighter, longer holding foam with smaller bubbles is a better quality cue than a tall, airy foam that collapses in seconds.