Deposition at room temperature of Ga on Si(100) produces single-atom-wide metal rows orthogonal to the Si-dimer rows. Detailed analysis using scanning tunneling microscopy reveals a monotonically decreasing size (i.e., length) distribution for these rows. This is unexpected for homogeneous nucleation without desorption, conditions which are operative in this system. Kinetic Monte Carlo simulation of an appropriate atomistic model indicates that this behavior is primarily a consequence of the feature that the capture of diffusing atoms is greatly inhibited in the Ga∕Si(100) system. The modeling also determines activation barriers for anisotropic terrace diffusion, and recovers the experimental distribution of metal rows. In addition, we analyze a variety of other generic deposition models and determine that the propensity for a large population of small islands in general reflects an enhanced nucleation rate relative to the aggregation rate.