Well, downsizing is a standard way to reduce noise. For a camera raw file it can be done by asking for the type of interpolation that just combines the RGBG pixels into one “RGB” pixel, producing a smaller but cleaner image. I haven’t ever used that type of interpolation. But in GIMP there is “linear” interpolation, that seems basically to do something like taking the median of the pixels surrounding each pixel (including the actual pixel). Asking for 50% reduction in width and length does produce a much cleaner image, at least when starting from a non-demosaiced image (it was an experiment, and worked really well as the starting file really was very noisy).
This “binning” - and maybe I’m using totally the wrong word, but hopefully I’ve explaing what I’m trying to point to by using the word - is different from using a scaling algorithm that tries to preserve detail. The assumption is that noise and detail look an awful lot alike to a scaling algorithm, so if size reduction while also decreasing noise is the goal, don’t use algorithms that try to preserve detail.