Reverting images from Photo's back to original file size and quality.

File -> Export -> Unmodified Original

That gets out what you put in.

Exporting the same format simply gives you the photograph in the same kind of file.

Size refers to the dimensions of the image - length by breadth, measured in pixels.

File size is determined by the Quality setting. It is, paradoxically, possible to export a file larger than the one you imported in some cases.

To understand how that can happen you need to understand that the Jpeg is not your photo, it's just a compression format applied to your photo. The relationship of Jpeg to photograph is the relationship of shoebox to shoes. Similarly, that Word file is not your novel, it's a container for your work. When you view a jpeg what happens is the file is rapidly decompressed and you can see the photograph. If you then export from Photos (apart from the unmodified original) Photos creates a new file and puts your photo into it - and the size of that file depends on the settings in your export dialogue. The lower the quality the smaller the file. The higher the quality the larger the file. And sometimes, exporting at the Maximum quality setting results in a file larger than the one you imported. This is not because any data has been added to the photograph, just that Photos is applying less compression that your camera did.

The images in photos on my Mac

are not as large as the original JPEG’s when they were taken.

How have the photos been imported to Photos?

Have they been imported with "File > Import"? Have the photos been migrated from an iPhoto Library or Aperture Library?

Have you been running any cleaning applications to remove duplicates and save storage? Such cleaning applications can reduce the quality of your photos by keeping just the low quality preview of an edited version.

When I export unmodified the dimensions are the same as original but the file size is quite a bit smaller.

Smaller than what? Where is the file you're comparing it with? Photos (nor any of its predecessors) do nothing to an imported file. It just copies it in. So if it's exporting a 1mb file as unmodified then that's what was brought in.

There is no way to improve the quality of the photograph. But you can reduce the compression by exporting at maximum.

A_747s wrote:

The photo's that were originally RAW do not revert to RAW when selecting unmodified also.

Then they were not Raw when imported. There is no reverting involved. When you import a raw it always remains a raw. The master file is untouched. Your decisions are recorded in the database and a preview version made, but the master is never touched. If you export an unmodified original then you get out what went in. There is no scenario where importing a raw would yield a Jpeg. You either get a raw or you get a failed export.

You're not using drag and drop are you? Or shooting raw and jpeg pairs?

That suggests to me that what you're exporting there are the Jpegs and not the raws then.