Loading too small images

Can you please tell me are you using a retina image or are you change the image size?

No I am definitely not using retina images and no I have not changed the image size. The uploaded post images are 800×500.

Can you please enable Retina Images in AMP? So the issue will be fixed. Follow this tutorial: https://ampforwp.com/tutorials/article/how-to-add-retina-images-in-amp/

And let me know if the issue is solved or not.

Yes it is fixed. Just to point out that the correct image size could be used for a better pagespeed score but the website performs better than before so I think it is too much to ask. (also I do not know if it is theme related at some point)

Anyway, problem solved so I would like to thank you very much for your help!