Emailing from Photos

Size refers to the dimensions of the image. When you share an image to Mail using the sharing mechanism it produces a preview - a good-for-most-things version of the shot. If you want to send the original image, or send a full quality image of an edited shot, then export it from Photos to the Finder: file -> Export, and make your selections there. Then attach that to your email.

Exporting in full size creates the same smaller size file as Photos mail sharing. 12.4MB in Photos, 2.2MB after exporting

Sure, but what quality setting did you export at?

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

Quality refers to the amount of Jpeg compression applied to the image. Higher quality means less compression and larger file size. It can even lead to a situation where the file size is larger than the original. That just means the app applied less compression than your camera did.

There is no "full size" that relates to File size. Try export the image at Maximum quality. What's the resulting file size then?

This is quite a complex area, but I'll try explains best I can. Jpeg is not an image format. it's a compression format. It has more in common with zip or rar files than it does with tiff files. Your camera takes a snap and processes that into kind of tiff file. This is compressed into a Jpeg to save space. When you view a photo saved as a Jpeg what happens is the file is decompressed for you to view it. If you edit the file it is recompressed, in a lossy way. Some data is thrown away, but only if you edit. Viewing makes no difference. When you export from Photos and choose the quality a new file, containing a copy of the photograph is created and compressed into Jpeg format. But Apple's algorithms will not be identical to your cameras, and so some variation is inevitable.

What are you emailing the image for?

In video they often talk about production formats and delivery formats. Video files are so huge that once the edit is finished it's output to a m4v or whatever, which is heavily compressed, but the quality difference for the viewer is not noticeable.

If you're sending this image to be edited, or for very high quality printing, then file size is important. Other than that, the Jpeg algorithms are so good that - with a well exposed shot - you can't tell if you're viewing a 2mb file or a 20mb file. And, if you're sending the image for editing, perhaps you'd be better not working with Jpeg at all. Also, with a well exposed shot, quite frankly, there is no guarantee that a 20MB file will print any better than the 2MB version.

Again: size refers to dimensions - length by breadth of the of image, measured in pixels.

Quality refers to the amount of compression. Higher quality, less compression, more the file 'weighs' measured in kb or MB

That is perfectly possible with a well exposed shot.

With Photos 5.0 there's an option in mail to send the Actual size:

Which will give you the actual file: