Blogger automatically converts your images into various sizes. If you do not select "Original size" when adding an image to a post, Blogger will select a small size which is not suitable. So even if you upload a large image, Blogger may have converted it to a much smaller size without you knowing. To avoid issues, you should always select "Original size" when adding images to posts.

To set an image as "Original size", go to the post editing screen. Then left-click on any image, and select "Original" from the options shown below:

Even if this makes the images too big for the post editing screen, they will be scaled to fit the template perfectly.