In this paper, the effect of the active fiber's core/cladding area ratio on the output parameters of 1018 nm fiber lasers has been investigated. In this regard, we conducted a comprehensive study of two fiber lasers that utilized 25/400 and 30/250 µm ytterbium-doped fibers (YDFs), both theoretically and experimentally. The optimum length of YDFs required for 40 dB of amplified spontaneous emission suppression was calculated. Theoretical studies also identified the YDF breaking zone for lengths greater than the optimum. The experimental results showed that selecting the proper dimensions and coiling diameter for the active fiber significantly increased the power and efficiency of the YDF laser. We obtained an output power of 943 W with a 75.5% slope efficiency for the co-pumped 30/250 µm YDFL which, to the best of our knowledge, is the highest reported value for the 1018 nm co-pumped fiber laser. An analysis of the experimental and theoretical results revealed that YDFs with a core/cladding area ratio greater than 1% are more suitable for realizing a high-power 1018 nm fiber laser. The findings of this study are crucial for the development of high-power 1018 nm fiber lasers with improved performance.