JPG vs compressed JPG vs WEBP - why WEBP isn't the smallest one?
I have this image (photo taken by me on SGS 9 plus): Uncompressed JPG image. Its size is 4032 x 3024 and its weight is around 3MB. I compressed it with TinyJPG Compressor and its weight was 1.3MB. For PNG images I used Online-Convert and I saw webp images much more smaller even than compressed with TinyPNG. I expected something similar, especially that I read an article JPG to WebP – Comparing Compression Sizes where WEBP is much smaller that compressed JPG. But when I convert my JPG to WEBP format in various online image convertion tools, I see 1.5-2MB size, so file is bigger than my compressed JPG. Am I missing something? WEBP should not be much smaller than compressed JPG? Thank you in advance for every answer.
Answer
These are lossy codecs, so their file size mostly depends on quality setting used. Comparing just file sizes from various tools doesn't say anything without ensuring images have the same quality (otherwise they're incomparable).
There are a couple of possibilities:
JPEG may compress better than WebP. WebP has problems with blurring out of the details, low-resolution color, and using less than full 8 bits of the color space. In the higher end of quality range, a well-optimized JPEG can be similar or better than WebP.
However, most of file size differences in modern lossy codecs are due to difference in quality. The typical difference between JPEG and WebP at the same quality is 15%-25%, but file sizes produced by each codec can easily differ by 10× between low-quality and high-quality image. So most of the time when you see a huge difference in file sizes, it's probably because different tools have chosen different quality settings (and/or recompression has lost fine details in the image, which also greatly affects file sizes). Even visual difference too small for human eye to notice can cause noticeable difference in file size.