Image size in thumbnails

[Pooh]

As per screenshot a 3072x2045 jpg image was adjusted in Affinity Photo but when shown in XnViewMP it is 512x457 How can I get both to the same size?

Screen Shot 2018-09-06 at 14.32.58.png

[xnview]

you means the black border in thumbnail?

[Pooh]

I do not mean the black border. Both thumbnails have a different image size and I would like to know why this is an if I can do anything to make them the same size - or does it matter that the sizes are different?

Thank you

[xnview]

if the image files has different pixel size, the thumbnails will be different