Provided are isolated polynucleotides which comprise a nucleic acid sequence at least 80% identical to SEQ ID NO: 321, 1-320, 322-480, 793-2945 or 2946; isolated polypeptides which comprise an amino acid sequence at least 80% homologous to SEQ ID NO: 517, 481-516, 518-792, 2947-4662 or 4663, nucleic acid constructs comprising same, transgenic cells and plants expressing same and methods of using same for increasing yield, biomass, growth rate, vigor, oil content, fiber yield, fiber quality, abiotic stress tolerance, and/or nitrogen use efficiency of a plant.