Provided are isolated polynucleotides at least 80% identical to SEQ ID NOs: 1-479, 813-5173, 8511, 8513, 8515, 8517, 8519, 8521, 8523, 9096-9141 and 9142; and isolated polypeptides at least 80% homologous to an amino acid sequence selected from the group consisting of SEQ ID NOs: 624, 480-623, 625-812, 5174-7015, 7017-7021, 7024, 7026-8510, 8512, 8514, 8516, 8518, 8520, 8522, 8524, 9143-9177, such as the polypeptides set forth in SEQ ID NO:480-812, 5174-8510, 8512, 8514, 8516, 8518, 8520, 8522, 8524, and 9143-9177, 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, nitrogen use efficiency, and/or abiotic stress tolerance of a plant.