Saving Image from PSD to Jpg dramatically reduces size

What is going on, and is there a way to preserve the same size when exporting to JPG?

By @craigpmhere

Just for context, the question is interesting because many people actually complain about the opposite:

“My 100K JPEG file became a 32MB Photoshop file!”

or

“My 8MB raw file became a 76MB TIFF file!“

In all cases the answer is the same: The size of an image file depends on things including:

Bits per pixel

Number of channels

Is it using compression?

If it’s using compression, is it lossless or lossy?

If it’s lossy, what compression/quality level was selected?

In the case of your Photoshop file vs the JPEG version, the answer is the Photoshop file uses lossless compression, and JPEG uses lossy compression.

Many people intentionally use JPEG because it can make a Photoshop file size small enough to post on the web or send in email. For them the dramatically reduced file size is the specific reason they chose JPEG.

If you are concerned about the file size becoming that small, what is your reason for converting it to a format other than Photoshop format? Do you need to preserve more quality, or layers, or something along those lines? Maybe there is another format that would better meet your requirements.

When I open the file, and then export is as a JPG, highest quality of "7"

7 is not the highest quality. 

Size and weight is not the same thing.  Why should a JPEG which is compressed have the same weight than a PSD file?

If you mean size just check the pixels wihich are the only way to measure an image.

Note than sometimes large PSD file with small size are use smart objects. If you open the smart object and check its size you will be surprised not to have the same pixel width than the PSD.

You may work on a 1920 px width size in which a smart object integrate a 5000 px width.

Recently I recovered 5 Go on my SSD just by changing smart objects sizes to match PSDs sizes…

😉

Jpeg compression is incredibly effective, which is why it still exists despite its serious drawbacks.

It can shrink a file down to 2 - 5 % of native size, with very little immediate visual degradation. But it's there, and with repeated saving the file will eventually disintegrate. Good for one-off final delivery to save bandwidth, but not suitable as a working archive format.

"preserve the same size when exporting to JPG"

No, there isn't. JPEG is a format that reduces file size while maintaining visual quality. That's one of the reasons why JPEG exists in the first place. JPEG uses lossy compression, which means it reduces the image quality every time you save it. You can find more information about compression and JPEG format online.
If you want to save your image with maximum quality and compatibility, you can try using TIFF. It is a format that most applications support, but web browsers do not.

JPG is compressed, thus a smaller file size.

ps. I have a large selection of files with the same basic situation, so it's not isolated to this one file.