We study the aggregation of diblock copolymers in a selective solvent as a function of the asymmetry of the copolymer chains. When the shorter block is in a poor solvent, our mean-field approach predicts copolymer micellization in spherical or cylindrical geometries. For large collapsed blocks a dense lamellar phase is formed in an excess of solvent. A further increase of the size of the molten blocks leads to the formation of reverse micellar structures, where small spherical swollen cores are surrounded by a majority matrix of molten chains.