Wetlands are important contributors to rising atmospheric CH₄ concentrations which can result in positive climate feedback. However, estimates of global wetland CH₄ emissions remain highly uncertain partly due to large uncertainties in wetland inundation area. The current global carbon project-CH₄ budget is built upon coarse spatial resolution (e.g., 0.25 degree) wetland area data; therefore, the impact of small wetlands on the magnitude and trend of global CH₄ emissions remains unclear. Here we quantified global CH₄ emissions from different sizes of small wetlands as well as their contributions to the trends in CH₄ emission changes during 2002-2022 regionally and globally. Specifically, we leveraged high-resolution (~30 m) satellite wetland maps, worldwide eddy covariance and chamber observations of CH₄ fluxes, and a causality-guided machine learning model for global wetland CH₄ emission estimates. We estimate that small wetlands (<1 km² in size) account for ~28% of the global total wetland area. Furthermore, wetlands smaller than 0.09 km² and 0.01 km² constitute an extensive portion of the total area of wetlands that are less than 1 km², indicating that 300 m and 100 m resolution remote sensing images are insufficient and higher resolution satellites (e.g., 30m resolution) are needed to explicitly delineate such fine-scale wetland extent. During the past two decades, substantial CH₄ was emitted from these small wetlands. More importantly, even when accounting for the uncertainty in the interannual changes of wetland area, increases in CH₄ emissions from small wetlands were robustly identified due to robust increases in emission intensity. Our results highlight the important but under-appreciated role of small wetlands in the global CH₄ budget and potentially in the recent increase in atmospheric CH₄ concentrations.