The compression ratio depends on the entropy of the image more than anything else. Chroma subsampling isn't the reason for low bitrates - by your own logic, gr

The compression ratio depends on the entropy of the image more than anything else. Chroma subsampling isn't the reason for low bitrates - by your own logic, grayscale would be smaller since it's effectively removing any chroma information. What gets you from 24 bpp to 0.12 bpp is primarily the arithmetic coder and that only depends on entropy. Just for shits and giggles I took two images (resized and cropped to 1280x720) and saved them as jpeg (15%, 4:2:0). The high entropy image (a garden with lots of flowers [1]) resulted in 83.476 bytes. The low entropy image (a stylised drawing of hills with a sunrise [2]) resulted in 22.299 bytes using the same settings. To further confirm the hypothesis, I applied a size 4 average blur to the low entropy image to further reduce details and saved it using the same settings. The result was a 19.104 bytes file. Doing the same to the high entropy image resulted in a 34.646 bytes file - a whopping 60% reduction in size by just reducing the entropy. This shows that reducing entropy is the significant factor here, not colour. It also means that you can get some 100x100 images to compress down to 144 bytes, but for most that simply doesn't apply. [1] https://images.hdqwalls.com/wallpapers/sunrise-landscape-minimalism-5k-ex.jpg [2] https://homesfeed.com/wp-content/uploads/2015/08/wonderful-landscaping-ideas-for-front-of-house-and-backyard-with-cool-walkways-and-colorful-flower-plants-and-trees.jpg