Thanks for the link, more good info.
Now that I think about it, there may be other factors that would influence target image sizes besides fish movement and the slant distortion and target compression listed in the tutorial.
Any skewing of the boat path would tend to expand/compress target size as would the angle of the fish to the sonar signal (but perhaps only in the extreme). For example, a large fish directly facing the signal might show a very small return image.
Harry