Spreading of homogeneous mixtures of bottle-brush and linear macromolecules of poly(n-butylacrylate) on a solid substrate has been monitored on the molecular scale by atomic force microscopy. Despite the nearly identical chemical composition and similar molecular weight, brush-like macromolecules move markedly slower than linear chains. Moreover, smaller bottle-brushes have been shown to flow faster than the larger bottle-brushes, resulting in fractionation of the macromolecules along the spreading direction. This behavior was explained by the difference in sliding friction coefficient between the bottle-brush macromolecules and linear chains with the substrate. A theoretical model of molecular size separation is in a good agreement with experimental data.