Unfortunately I’ve not seen this issue before so I can’t really tell what’s going – especially they were working before.
All I can tell is that the images are being uploaded very small in the actual post.
I still suspect that a third party plugin may have caused this – are you using any lazy load related plugins? If so maybe check with their support?
Have you checked the instructions here?
https://codex.wordpress.org/Inserting_Media_into_Posts_and_Pages
You should be able to select the image size when inserting into the content. If you did select the right size but it’s still not working then you will have to check with WordPress.
Sorry I couldn’t be more helpful.