images in Photogallery are too big (kb) compared to the uploaded originals

I got the question, why are the uploaded images to photo gallery got much more kb than the originals, uploaded from WordPress medialibrary.
The originals are all 1300px by 900 px maximum.

I set “0” at “Image dimensions” to keep the original pixel-size.
Although the uploaded images don’t have the right size, they are smaller (px) than I set in preferences and although they got more kb than the originals???

Thanxx in advance
znello

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