  The James Webb Space Telescope (JWST) discovered several luminous
high-redshift galaxy candidates with stellar masses of $M_{*} \gtrsim 10^{9} \,
\rm{M_{\odot}}$ at photometric redshifts $z_{\mathrm{phot}} \gtrsim 10$ which
allows to constrain galaxy and structure formation models. For example, Adams
et al. identified the candidate ID 1514 with $\log_{10}(M_{*}/M_{\odot}) =
{9.8}_{-0.2}^{+0.2}$ located at $z_{\mathrm{phot}} = 9.85_{-0.12}^{+0.18}$ and
Naidu et al. found even more distant candidates labeled as GL-z11 and GL-z13
with $\log_{10}(M_{*}/M_{\odot}) = 9.4_{-0.3}^{+0.3}$ at
$z_{\mathrm{phot}}=10.9_{-0.4}^{+0.5}$ and $\log_{10}(M_{*}/M_{\odot}) =
9.0_{-0.4}^{+0.3}$ at $z_{\mathrm{phot}} = 13.1_{-0.7}^{+0.8}$, respectively.
Assessing the computations of the IllustrisTNG (TNG50-1 and TNG100-1) and EAGLE
projects, we investigate if the stellar mass buildup as predicted by the
$\Lambda$CDM paradigm is consistent with these observations assuming that the
early JWST calibration is correct and that the candidates are indeed located at
$z \gtrsim 10$. Galaxies formed in the $\Lambda$CDM paradigm are by more than
an order of magnitude less massive in stars than the observed galaxy candidates
implying that the stellar mass buildup is more efficient in the early Universe
than predicted by the $\Lambda$CDM models. This in turn would suggest that
structure formation is more enhanced at $z \gtrsim 10$ than predicted by the
$\Lambda$CDM framework. We show that different star formation histories could
reduce the stellar masses of the galaxy candidates alleviating the tension.
Finally, we calculate the galaxy-wide initial mass function (gwIMF) of the
galaxy candidates assuming the integrated galaxy IMF theory. The gwIMF becomes
top-heavy for metal-poor starforming galaxies decreasing therewith the stellar
masses compared to an invariant canonical IMF.
