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Hey Jo,
Don’t worry, you’re not doing anything wrong!
(Warning: computer nerdary ahead 🤓)
The exported JPEG file should be much smaller than full fidelity raw files like ORF in your case, DNG or TIFF, by design. JPEG is a compressed, “lossy” format, which should end up representing the same image in a much smaller file size, with almost no noticeable loss of detail. (In reality, raw file formats like ORF/DNG/TIFF/etc hold way more detail than can be represented on screen, which is why we can make so many adjustments to exposure, white balance, etc with no loss in quality, before exporting.)
The other difference you see, with Lightroom reporting the size of your raw file as 16.27 MB while Finder reports 17.1 MB, is sadly an age old confusion in computer science. The debate is whether 1 KB equals 1000 bytes or 1024 (a power of 2) bytes. They are both arguably “correct”, depending on the math the programmer decided to use. Although multiples of 1000 was generally standardized – https://en.wikipedia.org/wiki/Gigabyte#Definition
The Mac’s Finder uses a simple assumption that 1 MB equals 1,000,000 bytes. The screenshot shows the actual size of the file is 17,060,475 bytes. The math that Finder uses to summarize this is 17,060,475 / 1,000,000 = 17.0605 MB or 17.1 MB.
Lightroom apparently assumes that 1 MB equals 1024×1024 bytes or 1,048,576 bytes. So the math it uses is 17,060,475 / 1,048,576 = 16.27 MB.
In summary, programmers should try harder to make computers less confusing 🙂 But I hope that helps explain what you’re seeing.
Cheers,
Chris