Once upon a time it was crazy uploading higher res' images, primarily intended for printing, to be viewed on lower res' CRT screens but that's what people used to do without thinking. And the cost of storage provision on web servers to facilitate those oversized files was much higher. I suppose that's why the sites' operators became so mindful and set limits. It's extraordinary how file sizes grow as you increase the resolution, although compression techniques can mitigate for that to a certain extent.
The problem now with modern flat screens and camera facilities, with their much higher usable resolution capabilities and other associated technology they use (e.g.editing that adds more data), you are going way beyond what was the norm for the file sizes being habitually created and transmitted way back when. I suppose the limits that were/are imposed for uploading by some sites are a hangover from an era where the devises and what they were going to be viewed on were more limited and storage costs were higher. It's just moving with the times and what is now the norm. Max