Although several models have been proposed to account for how cytoskeleton polymerization drives protrusion in cell motility, the precise mechanism remains controversial. Here, we show that, in addition to force exerted directly against the membrane by growing filaments, the way elongating filaments pack also contributes to protrusion by generating an expansion of the cytoskeleton gel. Tomography shows that filament packing in the major sperm protein (MSP) -based nematode sperm-motility machinery resembles that observed with rigid rods. Maximum rod-packing density decreases dramatically as the rods lengthen. Therefore, as filaments elongate, the cytoskeleton gel expands to accommodate their packing less densely. This volume expansion combines with polymerization to drive protrusion. Consistent with this hypothesis, an engineered MSP mutant that generates shorter filaments shows higher filament-packing density and slower movement.