unreal engine4-27 pixelstreaming the 2560*1440 resolution uses less traffic than the 1920*1080 resolution

During the traffic statistics, it is found that the resolution of 720405 gradually increases to 19201080, which is a normal incremental flow effect. When the resolution is higher than 1920*1080, the higher the resolution, the less the flow is used. Does anyone know the reason? The switch between frameWidth and frameWidth in webrtc monitoring is indeed successful, but the traffic is reduced instead. Does anyone know the reason.