SOLVED Upgrading system and photo pictures are downgraded (smaller size)? How come?

[Alfettaracer]

It happened to me that after upgrading to a newer version (for instance from El Capitan to Sierra) that my pictures were downgraded in terms of picture size.
Sometimes only a very small picture was left on my Mac in Photos (in my iPhoto Library) after upgrading. How can I restore this (if it all possible?) and how can I avoid this will happen again if I switch to Catalina.

any advice will be appreciated, because I don't want to loose my photo pictures quality this way.

 

[Lufbrarunner]

I am not sure why that would happen, but as insurance against a future occurrence, I would back up the photos to an external drive and re -import them after upgrading.

 

[Alfettaracer]

Thx, I understand this (as a matter of fact I made a backup earlier) But what is the cause of this problem, does anybody has a clue?
And do you know an efficiënt procedure to import a large library from an external drive into your (I) Photo library?