  Transiting planets with radii 2-3 $R_\bigoplus$ are much more numerous than
larger planets. We propose that this drop-off is so abrupt because at $R$
$\sim$ 3 $R_\bigoplus$, base-of-atmosphere pressure is high enough for the
atmosphere to readily dissolve into magma, and this sequestration acts as a
strong brake on further growth. The viability of this idea is demonstrated
using a simple model. Our results support extensive magma-atmosphere
equilibration on sub-Neptunes, with numerous implications for sub-Neptune
formation and atmospheric chemistry.
