When you insert them you can choose the size – you are currently choosing the medium option – try the full size option.
OK my bad, this option is not available with most themes – you will need to change the template file – probably single.php to pull the full width thumbnail instead of the medium width one – if you are not a coder you will need to get someone to do this for you. You can read about it here:
https://developer.ww.wp.xz.cn/reference/functions/the_post_thumbnail/
There may be a setting somewhere in your theme that could do this for you, it is hard to tell without more information.
