In unconsolidated sediments, some grains form a load-bearing framework while others may “float”, that is, occupy volume without contributing to mechanical strength. The latter grains affect the relationship between acoustic velocity and porosity as well as the permeability of the sediment. We propose that characteristic features of the grain-size distribution are indicators of the fraction of floating grains. The criterion for “floating” involves grain-scale geometry not accessible from macroscopic models of grain packing, so we support this proposition with an examination of computer-generated model sediments (dense, disordered packings of spheres) having two sizes of spheres. The analysis reveals two thresholds. First, the fraction of floating grains is larger when the volume fraction of small grains is below ~ 40%. In packings satisfying the first threshold, a second threshold exists: if the large-grain radius is sufficiently greater than the small-grain radius (by a factor of 3, or half a decade on a logarithmic scale), the fraction of floating grains is significantly larger. These thresholds are qualitatively consistent with field observations of anomalously slow P-wave velocities in several poorly consolidated to semiconsolidated reservoirs. The grain-size distributions in these reservoirs are sufficiently broad to satisfy the two thresholds, and the emergence of a second mode in the distribution correlates with greater anomaly in the velocity–porosity trend. The maximum fraction of floating grains in the model sediments is comparable to the value needed to explain the anomaly quantitatively. Significantly, the thresholds for floating grains are closely related to those known to affect trends of macroscopic properties of sediments (porosity, permeability). The grain-scale model can thus serve more generally as a microstructural complement to established macroscopic models.