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How to convert Samsung motion photo to still photo without loosing quality

I have received several photos taken with my friend’s Samsung phone.
Windows Photos app tells me that most of them are motion photos.

enter image description here

Now, I want to extract the original photo without losing quality. As far as I understood, the only image in a motion photo is the last one. I tried different methods to remove the motion. An original motion photo (12 MP) is around 9 MB. My own phone and Samsung gallery can only take HD screenshot. Google Photos extracts an image with 1.3 MB. Editing and saving photos via Windows Photos app generates a file size of 1.09 MB, while the option changing size leads to a file size of around 5.7 MB. Online website produced an even smaller file with 400 kB. Proposed method from this question ended up in unchanged files.

I’m wondering how to compare the files and find out which one holds the original quality photo? And is there an easier way to convert the photos (Windows/Android) I haven’t tried yet?