Thanks. It turns out the other 2 images were the same size coincidentally. The original size of the 2 images that are same was a lot bigger but the same. Hence why I thought there was another setting g I perhaps had to change.
So it turns out the images sizes have to be the same in order for their size to be reduced to the same proportion in the thumbnails.
So what is the thumbnail setting under media for in that case? Thanks again