The rapid coassembly of linear and linear dendritic amphiphiles from homogeneous solution and high supersaturation produces kinetically arrested nanoparticles, the morphologies of which are distinct from equilibrium structures. The binary system of poly(D,L-lactide-co-glycolide)-block-poly(ethylene glycol) with a linear or dendritic architecture of the hydrophilic component, forms spherical hybrid nanoparticles regardless of dendron generation or poly(ethylene glycol) length. Controlled variation in nanoparticle size was achieved through a balance of amphiphile architecture, blend composition, and final solvent content. These results demonstrate how kinetic features of the assembly process influence the formation of hybrid copolymer nanoparticles.