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JPEG compression confusion

Update:11-30Source: network consolidation
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Please help me to prove that I'm not crazy.
I recently got photos developed from 35mm film at a local "professional" store. When I looked at the files on my computer, I was confused to see that they were only around 500 KB each.
When I asked the shop about this, they told me that JPEGs are compressed. When you open a JPEG in Photoshop, the program uncompressed the JPEG. At that point, you can see the actual file size of the photo by going to Image > Image Size and looking where it says "Pixel Dimensions."
Is this true? From what I understood, JPEGS are compressed, but there's no such thing as a program "uncompressing" a file when it opens it. Getting a 500 KB file means I have a 500 KB file. That's that. The end.
Am I completely wrong in this regard? Please help!

The Best Answer

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But that file size in Photoshop, that means nothing in terms of the 500 KB file, right? Whether or not the actual JPEG is "uncompressed," that doesn't increase the file size, right?
That 4.52 MB is simply an estimate of what the file size would be *if* the file were saved as a Photoshop document, right?
In the end, do those pixel dimensions at 72 DPI feel like quality scans from a supposedly "professional" shop?
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