Pixel size and file size are only related if the file is uncompressed. PNG & JPG are both compressed formats so it depends on what file type you save as. Generally we like JPG for artsy maps like these or PNG for diagrammatic / vector style maps. We do support SVG format too tho.

You know we should redo the challenge we once did which was to make any kind of map you like but it has to be at least 1024x768 size and no more than 64Kb.

We all learned a lot about file compression out of that one.