Why does the image quality depend on the source of the ImageViewer?

Hey Forum! Have a look at the images below; (how) can the dramatic difference in quality be technically explained ? Looking forward to your answers! Thanks!  
1 answers

I think it's because in the Image view for 1 you have "Show" set to "Thumbnail" and on 2 you have "Show" set to "Full image". You probably have a pixels width or height set so 1 is being stretched to fill this with fewer pixels than 2. 

You can tell if you are using a thumbnail or a full image in the modeller by the size of the icon. If you look at 2, you'll see it's larger than in 1.