A quality-quantity trade-off selects for the absence of biological mass
The net energy that is available for self-replication generates a quality-quantity trade-off where a parent can produce many small or a few large offspring. And with fitness being given by the exponential increase in numbers on a per generation time-scale, r = ln λ ∝ ln [p ε τ / w], this generates a physiological background selection for the absence of mass, with the partial selection gradient on mass on logarithmic scale being minus one
∂ r / ∂ ln w = - 1
[λ:per generation multiplication factor; p:probability to survive to reproduce; τ:reproductive period; w:body mass].
The natural selection of organisms with anything but an absolute minimum mass is therefore, in one way or the other, dependent upon mechanisms that will make the intra-specific dependence of net energy on mass increase stronger than linearly with mass.