Schematic illustration of the comparison between the sizes of colliding particles and their mutual Hill sphere. When the Hill radius of a particle is smaller than its physical radius, particles colliding onto it rebound before they enter the Hill sphere, thus gravitational accretion is prohibited (left), while gravitational accretion is possible for colliding pairs of arbitrary size ratios when their mutual Hill radius is sufficiently large (right). In the intermediate case between the above two cases, gravitational accretion depends on the size ratio, and large particles can accrete small ones (middle).