Photos show different size when read from Mac Vs PC

Hi there,
I exported my pictures from my IMac Fotos (chossing the photos to export as original) to have them moved to my new Windows laptop. Now I was shocked when I accessed Mycloud through the explorer on my laptop and it showed the photos only in thumbnail sizes saying they have a size of around 7-15 KB. If I access the same photos on Mycloud through the Mac again, it shows them in full size and in the finder/file information with up to 5 MB per image. Does anyone know why is that and how I can get the full size displayed on the windows laptop?