This is because by default we set the width to 100% in the single post page so that it covers all the space there.
Bellow is the css that we use:
.single .featured img {width:100%;}
You can adjust the width accordingly.
Sorry I forgot to mention the place where you should add the css.
Do not make any changes in the files instead add the custom css in your appearance->customize->custom css:
OR
Use a child theme like blank child theme and add your custom css in the style.css file of the child theme.