Best quality photos when exporting?

If you're shooting just JPIEG files the Maximum Quality will be larger than the original JPEG file for some odd reason. I've never see an explanation for that. The unmodified original gives you exactly what the camera generated and downloaded to your library.

However, to move your library to a new computer there's no exporting too do. Do just what Lanny suggested and use Migration Assistant to move your files from the old Mac to the new Mac. However, before doing anything with the files on the old Mac be sure to test the library on the new Mac to make sure all is well and operation as expected.

If I choose unmodified original, I get a lower size photo than choosing the maximum quality photos.

What do you mean by "lower size"? Do you mean the file size? The pixel size is more important, and also the amount of JPEG compression.

Exporting a photo version at maximum size will usually increase the file size way beyond the size of the original image, if the original image is a highly compressed JPEG image. The problem is, that the JPEG compression is lossy. If you export a highly compressed JPEG with the highest quality, you will just waste disk storage. The quality, that already has been lost by the previous compression cannot be recovered. The best you can get is really exporting the original you previously imported.

For example, (a non-edited photo) Unmodified Original is 1.2 MB and Maximum Quality export is 4.7 MB. Another unedited photo is 3.8 MB when exporting Unmodified Original and 10.4 MB when using Maximum.

A bit of a layman's version, but accurate I think:

When you take a photo your camera processes the data from the sensor and basically interprets that data into a photograph. That photograph is compressed and stored as a Jpeg file. It's worth noting here that Jpeg is a compression format, not a graphic one. In fact, when you view a photo the Jpeg is decompressed to view the image.

The size of the Jpeg depends on how much data is in the photograph. A photo of a blue sky with no clouds will have less data than one of a wildly flowering garden with lots of flowers in different colours. Because the same amount of compression is used, regardless of the photo, the two Jpegs will be different sizes.

Now to exports:

Remember that when you view a Photo it is decompressed. So, view it, edit it and export it as a Jpeg. Why is the resulting file bigger? Because when you export at maximum quality Photos uses less compression than your camera did.

So, variation in files from the camera is a reflection of the amount of data in the photo.

Variation in the size of the files after export is because Photos used less compression than the camera.

So, same photo, different file sizes.

You got your words a little mixed up. The Unmodified Original is just that, everything your camera captured. Those files should be the largest. Edits are often converted to smaller sized jpeg files. They will be smaller because during the default compression process, data is thrown away to make the files smaller.

The best would be, Unmodified Original, but then you would lose your editing. The next best would be to just export the photos as you have edited them.

Then your editing is saving your edits in a different format than your originals. If your originals are jpeg, and you're using Photoshop as an editor, then you're likely saving the edits as TIFF or PSD files, uncompressed formats larger in size. Check the default settings that you're using in Photoshop. If you did the editing in Photos instead of Photoshop, the file sizes would have remained about the same.

You should probably take a course in photo editing to learn about the different file formats.

Your editing is never going to add anymore detail than what was in the original file. Your original file is going to be your best quality.

Don't export at all, just copy your Photos library to the new Mac. If you use Setup Assistant the first time that you turn on your new Mac, it will copy all of your stuff including your pictures.