In order to obtain a large-mode-area fiber structure to improve fiber lasers′ output power, several technical measures have been adopted to scale the fiber mode field size. However, with the mode field size increasing, the fiber waveguide′s ability to confine the mode field declines, which limits the growth of the mode field size. Based on the fiber eigen-mode field theory, the mode field distributions of the large-mode-area fibers are discussed, and the limitation effects of the mode field size growth imposed by mode coupling and mode distortion are analyzed. Considering the impact of the limitations, the mode field distributions of several refractive index fibers are calculated by using the finite element method. The mumerical results show that the product of the equivalent mode area and the square difference of the effective refractive index between two adjacent modes tends to a constant, which constrains, the scalability of the fiber mode field.