JPEG compression has some loss of quality and the outcome size depends on the algorithm in use.
I recomment the MozJpeg library as the best I've seen so far. It is able to generate images small in size yet with a very good quality.
Download the latest version binary from here: https://mozjpeg.codelove.de/binaries.html
For .NET use this wrapper: https://github.com/JosePineiro/MozJpeg-wrapper
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