Q: Is there a way to share photos in FULL size using iCloud photo sharing?

As you said:  iCloud Photo Sharing FAQ

iCloud Photo Sharing supports JPEG, TIFF, PNG, and most RAW photo formats. When shared, photos taken with standard point-and-shoot cameras, SLR cameras, or iOS devices will have up to 2048 pixels on the long edge. Panoramic photos can be up to 5400 pixels wide.

The Shared Photo Streams limit the pixel size of photos, so that the longest dimension - width or height, will have no more than 2048 pixels.

If you want to share full size photos, send them by mail at a full resolution and use Mail drop, if you are using Yosemite and it is working now (Mail (Yosemite): What’s new in Mail?).

Are you using Mail.app as Mail client or iPhoto  -your settings in the iPhoto Preferences "Email photos using ...."?

When I share a photo with the setting "general > Email Photos using Mail", it will have the same resolution as the edited version, a slightly smaller files size as the original image, but the same pixel size.

But I have to take care to correct the compression setting in Mail back to "Actual Size, Full Quality". Even if I select "Actual Size, Full Quality" in iPhoto, the Mail window will pop up with "large".

I notice too, for example, when i exported a 7.9MB iphoto jpeg with max full quality, it showed up in finder with a 15.7mb file size.

Yes, that doesn't make a lot of sense.  If you want to get the exact same image file out of iPhoto as you took and imported export it out via the File ➙ Export ➙ File Export  menu option with Kind = Original.  That gives you a bit by bit exact copy of the file. If you  add that file via the Finder to an email and select Actual Size you will send the same, original file.  If you use Mail as the email client and send the Actual Size image from iPhoto you should also get the original image file as long as it hasn't been edited.

But you have opened up a new can of worms - two files with same "pixel resolution" can have dramatically different file sizes? Is that because different (lossless) compression algorithms are used by different programs?

The "same pixel size" does not necessarily mean lossless compression. Only if you are using a lossless format like TIFF or RAW, but  a JPEG can have any file size at a given pixel size, depending on the "quality" value.

A JPEG is representing the image file by a mathematical approximation, and the quality slider is specifying how accurate the approximation will be. A  lower quality will need less coefficients for the approximation and result in a smaller file size, but will still have the same pixel size.

I notice too, for example, when i exported a 7.9MB iphoto jpeg with max full quality, it showed up in finder with a 15.7mb file size.

That can happen, if you edit a JPEG, that originally had a medium quality setting, and then export the edited JPEG at maximum quality.   The file size can increase, when the JPEG will be rendered at the highest quality and all JPEG artifacts in the original image will be perfectly approximated.  With JPEGS it really helps to know the original quality values, because it would be futile to export a JPEG of medium quality with the highest quality settings. That cannot bring the original quality back.