OK, I found the problem…
If the uploaded image is smaller than the crop size 600 x 600 then the image is not cropped or resized and breaks the catalogue grid.
Not sure how to approach this issue? I am sure there will be occasions when the shop admin is going to upload smaller images (which should not break the grid). The images may still have enough resolution to fit the size in the grid (eg the catalogue images size are set to 600 x 600 but the display on a large screen is only 262 x 262)