  The exotic range of known planetary systems has provoked an equally exotic
range of physical explanations for their diverse architectures. However,
constraining formation processes requires mapping the observed exoplanet
population to that which initially formed in the protoplanetary disc. Numerous
results suggest that (internal or external) dynamical perturbation alters the
architectures of some exoplanetary systems. Isolating planets that have evolved
without any perturbation can help constrain formation processes. We consider
the Kepler multiples, which have low mutual inclinations and are unlikely to
have been dynamically perturbed. We apply a modelling approach similar to that
of Mulders et al. (2018), additionally accounting for the two-dimensionality of
the radius ($R =0.3-20\,R_\oplus$) and period ($P= 0.5-730$ days) distribution.
We find that an upper limit in planet mass of the form $M_{\rm{lim}} \propto
a^\beta \exp(-a_{\rm{in}}/a)$, for semi-major axis $a$ and a broad range of
$a_{\rm{in}}$ and $\beta$, can reproduce a distribution of $P$, $R$ that is
indistinguishable from the observed distribution by our comparison metric. The
index is consistent with $\beta= 1.5$, expected if growth is limited by
accretion within the Hill radius. This model is favoured over models assuming a
separable PDF in $P$, $R$. The limit, extrapolated to longer periods, is
coincident with the orbits of RV-discovered planets ($a>0.2$ au,
$M>1\,M_{\rm{J}}$) around recently identified low density host stars, hinting
at isolation mass limited growth. We discuss the necessary circumstances for a
coincidental age-related bias as the origin of this result, concluding that
such a bias is possible but unlikely. We conclude that, in light of the
evidence that some planetary systems have been dynamically perturbed, simple
models for planet growth during the formation stage are worth revisiting.

Earth and Planetary Astrophysics;Solar and Stellar Astrophysics