Is Photo compression-ratio for JPEG exports too high?
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.