  In this letter, measurements of the shared momentum fraction ($z_{\rm{g}}$)
and the groomed jet radius ($R_{\rm{g}}$), as defined in the SoftDrop algorihm,
are reported in \pp collisions at $\sqrt{s} = 200$ GeV collected by the STAR
experiment. These substructure observables are differentially measured for jets
of varying resolution parameters from $R = 0.2 - 0.6$ in the transverse
momentum range $15 < p_{\rm{T, jet}} < 60$ GeV$/c$. These studies show that, in
the $p_{\rm{T, jet}}$ range accessible at $\sqrt{s} = 200$ GeV and with
increasing jet resolution parameter and jet transverse momentum, the
$z_{\rm{g}}$ distribution asymptotically converges to the DGLAP splitting
kernel for a quark radiating a gluon. The groomed jet radius measurements
reflect a momentum-dependent narrowing of the jet structure for jets of a given
resolution parameter, i.e., the larger the $p_{\rm{T, jet}}$, the narrower the
first splitting. For the first time, these fully corrected measurements are
compared to Monte Carlo generators with leading order QCD matrix elements and
leading log in the parton shower, and to state-of-the-art theoretical
calculations at next-to-leading-log accuracy. We observe that PYTHIA 6 with
parameters tuned to reproduce RHIC measurements is able to quantitatively
describe data, whereas PYTHIA 8 and HERWIG 7, tuned to reproduce LHC data, are
unable to provide a simultaneous description of both $z_{\rm{g}}$ and
$R_{\rm{g}}$, resulting in opportunities for fine parameter tuning of these
models for \pp collisions at RHIC energies. We also find that the theoretical
calculations without non-perturbative corrections are able to qualitatively
describe the trend in data for jets of large resolution parameters at high
$p_{\rm{T, jet}}$, but fail at small jet resolution parameters and low jet
transverse momenta.
