Provided are isolated polynucleotides encoding a polypeptide at least 80% homologous to the amino acid sequence selected from the group consisting of SEQ ID NOs: 757, 456-756, 758-774, 8385-10836, and 10838-14462; and isolated polynucleotide comprising nucleic acid sequences at least 80% identical to SEQ ID NO: 377, 1-376, 378-455, and 775-8384. Also provided are nucleic acid constructs comprising same, isolated polypeptides encoded thereby, transgenic cells and transgenic plants comprising 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.