I found meanwhile out:
The "normal" images have a size of: 900 x 675 pixels.
The recently shrunk images have a size of ONLY: 600 x 450 pixels.
The aspect ratio is in both cases: 4:3, thus no difference in the aspect ratio.
Original image file sizes are (larger than) 6000 x 4500 pixels, thus definitely not too small.
The shrinking phenomenon is very annoying for two reasons:
1) With their smaller display sizes, the images appear by far less impressive.
The area of the recently shrunk images is ONLY 2/3*2/3 = 44% of previous size!!!
2) The magnifying preview tool does not work. A potential buyer cannot check size and quality.
How can this be fixed?
I would be very grateful for a solution,
Andreas