Provided are isolated polynucleotides and nucleic acid constructs which comprise a nucleic acid sequence at least 80% identical to a nucleic acid sequence selected form the group consisting of SEQ ID NOs: 1-219, 367-5628, 9688-9700, and 9709-9752; and isolated polypeptides which comprise an amino acid sequence at least 80% homologous to an amino acid sequence selected from the group consisting of SEQ ID NOs: 220-366, 5629-9400, 9701-9708, and 9753-9796. Also provided are transgenic cells and plants expressing same and methods of using same for increasing yield, growth rate, biomass, vigor, oil content, seed yield, fiber yield, fiber quality, nitrogen use efficiency, and/or abiotic stress of a plant.