Is Photo compression-ratio for JPEG exports too high? Despite selecting the High option when exporting JPEG images from Photo, I am finding the file size at destination is around 3MB - 60% smaller than when I exported the same images from iPhoto with the High option. Image size is unaffected. If Photo is compressing JPEG images more during export than iPhoto, I fear the image quality is also being reduced. Is this correct?

There is a possibility that Photo is actually extra-compressing images at the import stage, not during export. Or both!
You can export unmodified original photos from the Photos.app from File > Export > Export Unmodified Original.

You can also select a higher resolution when exporting from File > Export > Export or cmd + ⇧+ E; and select "Maximum" from the drop down menu.
I suspect it is still recompressed, so it is expectable that the image will be smaller. 

Also, check this question and answers.