What are the factors on JPG compression ratio?
Despite there many obvious factors affecting image compression, since web images are intended to be displayed on an RGB display only, at a fixed resolution only (for bandwidth saving), the only relevant factor is the small detail in the subject, and the visible artifacts are only enhanced or lessen by the size they appear at, which is caused by the size of the pixel, or resolution of the display. Yes, the compression will leave visible artifacts which will be seen with the eye, and the smaller the pixel, the more difficult it would be for them to be noticed. The same image file would look really bad in 72ppi screens, but it’ll do fine in an iPhone’s which has a 160ppi resolution, and even better in a brochure at 300dpi just because the pixel, and so the whole image, would be 4 times smaller. At the same pixel count, the higher the resolution (pixels per inch), the smaller the pixel (more smaller pixels per inch), the more compression can be applied without noticing its artifacts.