Q: Full flash memory but storage is on external drive??

Hi!

I am trying to understand why my flash memory of my Macbook Pro early 2015 is full (mostly of photos) when my Photos library is placed on an external drive and on iCloud? A short background:  A couple of months ago I sold my iMac on which I stored my photo library on the internal drive. I therefore moved my library to an external drive and bought my new laptop.

So, yesterday I decided that I wanted to use my external hard drive as my library. That was no problem. Then I wanted to activate iCloud storage and NOT optimize storage BC I don't really trust the cloudy thing... Somehow, when I looked at my computer this morning, the computers internal memory became full in this process? The flash storage had a lot of empty space before and now it is full....

So, after that I decided to go with the optimize storage - option and hence moved my 700+ GB storage to iCloud (well in a few hours I have moved 12 GB so moving is still in progress) But my questions are:

- how come my internal memory is full of photos that I can't find on the computer (when looking in the Photos library in Finder it is empty, and points to my Photos program). I can't clean up the space?

- how come my storage is 120 GB but when I hold the mouse over Photos it says it contains 739 GB of data?

Well hopefully you understand even though it's in Swedish...

Help me!! I appreciate all input! I chose low storage on the internal memory since I wanted to keep my photo library off the computer!