They can support kilowatt levels of continuous output power because of the fiber's high surface area to volume ratio, which allows efficient cooling. The fiber's waveguide properties reduce or eliminate thermal distortion of the optical path, typically producing a diffraction-limited, high-quality optical beam.
Due to its large surface area relative to its volume, the fiber can effectively cool down and generate kilowatts of continuous power output. Its unique wave guiding properties also prevent significant heating and distortion of the light path, leading to a highly focused and clear optical beam.