How do I transfer my photos at full original resolution? Help!

Imac 27" (Retina 5K - late 2015)

Processor 3.2 GHz Intel Core i5

Memory 8GB DDR3

Macintosh HD 1TB-fusion

OS X El Capitan Version 10.11.6

Photos Version 1.5 (370.42.0) with 2 photo libraries

Hi,

I want to transfer pictures from one photo library to the other.

My efforts so far have consisted of exporting groups of pictures from photo library 2 to an external HD and then opening photo library 1 and importing those pictures from the external HD.

The problem is that the exported pictures have a much lower resolution (smaller file size) than the original to the point that they are unusable on a 27" screen.

I have tried changing settings for quality, colour and size in the export window but I cannot get it to export at the same resolution (file size) as the original : it either exports a file about 3x the size or a file of ...kB instead of ...MB

How do I get 'Photos" to export pictures the same size as the original ??

Northernblonde.