Thermal management is critical for kw-level power lasers, where mode instability driven by quantum defect heating is a major challenge. Tandem pumping using 1018nm fiber lasers are used to enable both high brightness and low quantum defect. It is, however, difficult to realize efficient 1018nm YDFL. The best demonstration to date is limited by the use of both conventional aluminosilicate host and smaller core diameters. In these cases, higher inversion is required due to the aluminosilicate host and higher pump brightness is required due to the smaller core, which results in high signal brightness for the same output power. These factors lead to large pump power to exit fiber, resulting in poor efficiency. Phosphosilicate host, on the other hand, requires much lower inversions to reach the gain threshold at 1018nm. The combination of phosphosilicate host and large-core leakage channel fibers (LCF) is a perfect candidate for efficient 1018nm fiber laser. We report a highly efficient Yb-doped phosphosilicate LCF laser with a quantum defect of 4.1% using a ~50μm-core diameter and ~420μm cladding diameter. The slope efficiency with respect to the launched pump power at 1018nm is 70%. The ASE suppression is <60dB. The large cladding of 420μm demonstrates a combination of high efficiency, ~4% quantum defect and high-power low-brightness diode pumping. We have also studied the limits of operating ytterbium fiber lasers at shorter wavelengths and found the efficiency to fall off at shorter wavelengths due to the much higher inversions required.