Board Thread:Support Requests - Designing Your Wiki/@comment-29644557-20190919204030/@comment-168424-20190920232742

FYI, regardless of what an app says the size will be on save, you should at least check the size that Finder/Get Info or File Explorer/Properties says the file is after you save it. Photoshop is super accurate, so I usually don't worry about it, but it is also very expensive and professionals don't put up with the BS that you get with free software, usually.

Also, TBH, I'm suspicious about you said about Preview, because I just tested it on my Mac and the saved file size wasn't nearly as in accurate as you got (298kb in Preview results in 347kb output file). I did Export... to JPEG in Preview which said 300kb and got a 301kb file and then did 282kb in Preview resulting in a 287kb file. Whatever you're doing, you're doing it wrong. The good news is you probably can use Preview in the future, just check the file size before trying to upload.