Most of my maps are in 8K resolution which does make for large file sizes, and it seems many map makers only provide 4K resolution.
File size if sometimes stated as a reason, the consumers find a large file size too unwieldy when using in a VTT program.
I haven't found this myself, although I have a fairly decent broadband connection and only really have experience of RPTools MapTool as a VTT product, but regardless I would like to say that I will continue to produce map to the highest resolution possible to maintain the quality of the image.
The difference is very clear in this close up of a section of the city of Vorulanium. The centre section is taken from the 8k file and the outer section a 4k version.
And just in case you didn't know both 'Paint' on Windows and 'Preview' on MacOS, are both built in apps that come with the OS and will very simply alter the file size to half or quarter the resolution for you if you do have any problem.