In most WordPress themes there are default sizes for specific images that are displayed on the page. This means that the original image is not necessarily displayed. You could be able to see from the image url whether it is original image or a cropped version.
On the other hand, if this cropping does not happen and it is the full image that is displayed, the end result will most likely look bad as well. 1MB image sounds awfully large for web use. If you take into consideration that most of us have laptops with a pixel density of 72 and resolution somewhere 1366 x 768 and are therefore unable to even view larger images.
If you would have shared a link to your site, I could have told you exactly what had happened in this case.