Reduction of channel width and area without major anthropogenic intervention has been a topic of interest in geomorphology as identified by some earlier studies, indicating contradictory phenomena in relation to the widely accepted downstream increases in channel width and area. However, there is limited knowledge regarding the downstream reduction of bankfull channel area and width (DRBCWA) in the absence of major anthropogenic factors in the existing scientific literature. Therefore, the current study presents a comprehensive review of downstream decreasing channel width and channel area in response to natural factors. In this study, synthesis methods were followed to achieve our goal. A significant amount of literature was systematically surveyed, and data related to downstream variations in channel size were compared and analyzed to fulfil our objective. This study found that DRBCWA is not limited to a particular climatic and geo-environmental setup; rather, it has been observed across a range of climates, including temperate maritime climates, arid zones, semiarid climates, and hyper-arid areas and over monsoon-dominated regions. Here, the loss of water from the channel in wide floodplain regions with cohesive bank materials and the lowering of sea level are found to be the key driving factors responsible for the reduction of channel width and capacity downstream.