  We aim at understanding the massive star formation (MSF) limit $m(r) = 870
M_{\odot} (r/pc)^{1.33}$ in the mass-size space of molecular structures
recently proposed by Kauffmann & Pillai (2010). As a first step, we build on
the hypothesis of a volume density threshold for overall star formation and the
model of Parmentier (2011) to establish the mass-radius relations of molecular
clumps containing given masses of star-forming gas. Specifically, we relate the
mass $m_{clump}$, radius $r_{clump}$ and density profile slope $-p$ of
molecular clumps which contain a mass $m_{th}$ of gas denser than a volume
density threshold $\rho_{th}$. In a second step, we use the relation between
the mass of embedded-clusters and the mass of their most-massive star to
estimate the minimum mass of star-forming gas needed to form a $10\,M_{\odot}$
star. Assuming a star formation efficiency of $SFE \simeq 0.30$, this gives
$m_{th,crit} \simeq 150 M_{\odot}$. In a third step, we demonstrate that, for
sensible choices of the clump density index ($p \simeq 1.7$) and of the cluster
formation density threshold ($n_{th} \simeq 10^4\,cm^{-3}$), the line of
constant $m_{th,crit} \simeq 150 M_{\odot}$ in the mass-radius space of
molecular structures equates with the MSF limit for spatial scales larger than
0.3\,pc. Hence, the observationally inferred MSF limit of Kauffmann & Pillai is
consistent with a threshold in star-forming gas mass beyond which the
star-forming gas reservoir is large enough to allow the formation of massive
stars. For radii smaller than 0.3\,pc, the MSF limit is shown to be consistent
with the formation of a $10\,M_{\odot}$ star out of its individual pre-stellar
core of density threshold $n_{th} \simeq 10^5\,cm^{-3}$. The inferred density
thresholds for the formation of star clusters and individual stars within star
clusters match those previously suggested in the literature.
