Supposedly this is our result list:
and we auto mark it like this:
All images whose file size are smaller on each group will be marked:
If we choose images on selected groups instead of images on all group like this:
We will get:
Only images on group no. 1 are marked, that’s because only the first image is selected (belong to group no. 1).
Now, if we want to auto mark images on group no. 1, 2 and 3. Select at least an image on those groups like this (press and hold Ctrl or Shift button while clicking to select multiple images):
Then auto mark it like before. The result is: