  Geometrical cross sections of dust aggregates determine their coupling with
disk gas, which governs their motions in protoplanetary disks. Collisional
outcomes also depend on geometrical cross sections of initial aggregates. In
the previous paper, we performed three-dimensional N-body simulations of
sequential collisions of aggregates composed of a number of sub-micron-sized
icy particles and examined radii of gyration (and bulk densities) of the
obtained aggregates. We showed that collisional compression of aggregates is
not efficient and that aggregates remain fluffy. In the present study, we
examine geometrical cross sections of the aggregates. Their cross sections
decreases due to the compression as well as their gyration radii. It is found
that a relation between the cross section and the gyration radius proposed by
Okuzumi et al. is valid for the compressed aggregates. We also refine the
compression model proposed in our previous paper. The refined model enables us
to calculate the evolution of both gyration radii and cross sections of growing
aggregates and reproduces well our numerical results of sequential aggregate
collisions. The refined model can describe non-equal-mass collisions as well as
equal-mass case. Although we do not take into account oblique collisions in the
present study, oblique collisions would further hinder compression of
aggregates.
