  This paper investigates the growth of the most massive cosmological objects.
We utilize the Simsilun simulation, which is based on the approximation of the
silent universe. In the limit of spatial homogeneity and isotropy the silent
universes reduce to the standard FLRW models. We show that within the
approximation of the silent universe the formation of the most massive
cosmological objects differs from the standard background-dependent approaches.
For objects with masses above $10^{15} M_\odot$ the effect of spatial curvature
(overdense regions are characterized by positive spatial curvature) leads to
measurable effects. The effect is analogous to the effect that the background
cosmological model has on the formation of these objects (i.e. the higher
matter density and spatial curvature the faster the growth of cosmic
structures). We measure this by the means of the mass function and show that
the mass function obtained from the Simsilun simulation has a higher amplitude
at the high-mass end compared to standard mass function such as the
Press-Schechter or the Tinker mass function. For comparison, we find that the
expected mass of most massive objects using the Tinker mass function is
$4.4^{+0.8}_{-0.6} \, \times 10^{15} M_\odot$, whereas for the Simsilun
simulation is $6.3^{+1.0}_{-0.8} \, \times 10^{15} M_\odot$. We conclude that
the nonlinear relativistic effects could affect the formation of the most
massive cosmological objects, leading to a relativistic environment-dependence
of the growth rate of the most massive clusters.
