Whatever the cause: file size is cannot be the goal. Jpg compression, a kind of "let's assume that" ,  works fine with smooth transitions like clouds and skin, but not with intricate patterns with sudden changes due to  differences in hue and contrast.
It's not a good attitude to say that something has a maximum size. Real life does not bend to such rules.
That's why a slight blur can help a lot.

Also, the save for web option is optimized and better in quality. Because on the web filesize needs to be as small as possible, it's normal that all unnecessary data are eliminated.
Of course you can always use Irfanview for jaypeggin'. Imo it outperforms PS quite often.

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