I have experienced the same issue when I changed my theme a while ago. The whole issue comes down to the size of the original image uploaded.
The previous theme I used used a large image size of 625px. This was the size I used to upload all my images. When I changed that theme, I found that the new theme uses a large image size of 724px. Unfortunately WordPress or even plugins like Regenerate Thumbnails does not upsize images. So images uploaded at 625px will have a maximum size of 625px.