Monopiles are the favoured foundation type for offshore wind turbines due to a relatively simple design and installation process. This paper investigates using coupled three-dimensional finite element analysis the load-deformation behaviour of extra-large (XL) and extra-extra-large (XXL) monopiles in low-shear strength over-consolidated Kaolin clay subjected to monotonic lateral loading at varying load eccentricities. The sub-loading tij constitutive model, robustly calibrated using data from centrifuge tests carried out by the authors, has been adopted in these analyses. For the 16 cases analysed, a detailed discussion is provided on the behaviour of XL/XXL monopiles, including their mode of deformation, lateral capacity, deflection, rotational stiffness, bending moment & shear force, and the response of the clay in terms of soil resistance and excess pore pressure. It is also demonstrated that the p-y method is inaccurate in designing monopiles in this size range.