  Constraining the formation processes of small solar system bodies is crucial
for gaining insights into planetesimal formation. Their bulk densities,
determined by their compressive strengths, offer valuable information about
their formation history. In this paper, we utilize a formulation of the
compressive strength of dust aggregates obtained from dust $N$-body simulations
to establish the relation between bulk density and diameter. We find that this
relation can be effectively approximated by a polytrope with an index of 0.5,
coupled with a formulation of the compressive strength of dust aggregates. The
lowest-density trans-Neptunian objects (TNOs) and main-belt asteroids (MBAs)
are well reproduced by dust aggregates composed of 0.1-$\mathrm{\mu}$m-sized
grains. However, most TNOs, MBAs, comets, and near-Earth asteroids (NEAs)
exhibit higher densities, suggesting the influence of compaction mechanisms
such as collision, dust grain disruption, sintering, or melting, leading to
further growth. We speculate that there are two potential formation paths for
small solar system bodies: one involves the direct coagulation of primordial
dust grains, resulting in the formation of first-generation planetesimals,
including the lowest-density TNOs, MBAs, and parent bodies of comets and NEAs.
In this case, comets and NEAs are fragments or rubble piles of first-generation
planetesimals, and objects themselves or rubbles are composed of
0.1-$\mathrm{\mu}$m-sized grains. The other path involves further potential
fragmentation of first-generation planetesimals into compact dust aggregates
observed in protoplanetary disks, resulting in the formation of
second-generation planetesimals composed of compact dust aggregates, which may
contribute to explaining another formation process of comets and NEAs.
