They used to be different. Back in the day, 'Share' meant a shrunken version and 'Copy' meant the full-sized photo.
Now they changed it so the mail program asks you how to size them AFTER you hit 'send.' So there's no longer a big difference between the two methods.
And thank god for that, I hated having to explain to people how to send me a photo every time I wanted one from them. Now I just say "send me the full-sized version" and they can do it without me showing them how.