In the WordPress Twenty Sixteen theme when the viewport is over 985px the single posts float to the right of the .entry-footer info which contains the author, gravatar, categories and tags and a link to the comments. They don’t go the full width at that particular size.
When the viewport is lower than 985px the .entry-footer goes below the post content in a inline style.
If you want to have this behaviour at any size you need to do some editing to the CSS file style.css
Find and remove….
And now they will stack.
To style the footer data as all inline at any size, find and remove the CSS below
Now the footer info will be inline