Adsorption is an important issue both in the estimation of natural gas reserves and efficient storage of methane. In this study, we focus on the mechanisms of methane adsorption in carbon nanopores and endeavor to establish the equation of state for the adsorbed phase through molecular dynamics simulations and theoretical analyses. Here, the nanopores were modeled by carbon nanotubes (CNTs). The higher storage capacity of the CNT compared to the bulk phase was attributed to the additional pressure exerted by the CNT wall on the adsorbed phase, considering which, the equation of state for the adsorbed phase was established. As the CNT diameter increases, the adsorption structure transforms from a single-file chain to two adsorption layers. Moreover, it was found that there exists an optimal CNT diameter that maximizes the adsorption, which is due to the competition between the curvature effect and the size effect. In the explanation of this phenomenon, the nanostructure of the CNT wall plays an important role, without considering which, the adsorption density would monotonically decrease as the CNT diameter rises. Our findings and related analyses may help reveal the underlying mechanisms behind the adsorption phenomena, which is not only of theoretical importance, but may also help estimate the natural gas reserves and design nanoporous materials with higher storage capacity.