Can I resize images without changing pixel size on my MacBook Pro?

Much of the size business is within your control. When you export a picture in Preview, you are asked what sort of file you want to use and what degree of compression you would like to use. I see that when I change the compression quality on an originally 31MB file, it can be saved with a size anywhere between 6MB and 128MB-- and that's all with the same compression format. With different formats, it can be as small as 0.35MB or as large as 350MB.

You don't say what you're choosing, and how this compares with the original file, but those things really affect the file size.

Sequoia 15.4.1 Preview.app > Tools > Adjust Size... has this issue:

With such new iOS 18 .heic without any adjustments the "Resulting size" remains original for a split second, and is then about halved. Press OK, and when Preview.app then closes, it re-saves and compresses the image to a smaller size (resolution remains the same) without asking if the user wants to save changes (bad!). And XMP metadata such as XMP-dc:Subject holding the Keywords is deleted (bad!) while XMP-dc:Description is preserved.

Edit: it is possible to revert that forced edit via Preview.app > File > Revert To. But that does not seem to work after that file is duplicated or maybe even after moved to another folder. It seems Preview.app is using a snapshot for the edit history. IMHO Preview should ask the user if s/he wants to save any edits when closing the file or quitting the app.

Yes, Sequoia 15.4.1 Preview.app > Tools > Adjust Size... has this issue:

It might be most apparent in the "new style" iOS 18.2 iPhone 16 Pro .heic, but the same happens also in old .heic and .jpg although there the change in file size is not so noticeable (resolution remains the same):

With such new iOS 18 .heic without any adjustments the "Resulting size" remains original for a split second, and is then about halved. Press OK, and when Preview.app then closes, it re-saves and compresses the image to a smaller size (resolution remains the same) without asking if the user wants to save changes (bad!). And XMP metadata such as XMP-dc:Subject holding the Keywords is deleted (bad!) while XMP-dc:Description is preserved.

Try unticking the "Resample image" checkbox in the Tools > Adjust Size… dialog before changing the size.

léonie wrote:… I will now never again allow Preview to touch my original files.

I use GraphicConverter ($40) for that sort of thing-- it's perfect, especially for those of us who already have it…

I'm not sure about the adjust thing in Preview. If I adjust and save, the file is smaller. If I then undo and save, it's original size again. I haven't used "Save" much--I like the Export function (is that the same as "Save as…" which disappeared?) that gives some control over the resulting file.

That's horrible, Keith. I have never noticed it, because I have only been using Preview's resize on screenshots for sharing and then saved the new version.

Now I can see that Preview is changing the file size of the image I opened, as soon as I press ok when I use Tools > Adjust Size. Even before I close the image and quit Preview.

Thank you for the warning. I will now never again allow Preview to touch my original files.

Added:Preview is already showing a smaller file size , as soon as I open "Adjust Size", even before I enter a new pixel size, just opening the adjust tool does it.

For example - this image has a file size of 9.9 MB.

Opening "Adjust size" will show a file size of 2.9MB at 100% !

After quitting Photos without saving anything the file size has been changed to 4.9 MB, surprise!