It is not uncommon when logging into WordPress admin to see visual anomalies ini the main content box for posts, pages and custom post types. A typical issue will be seeing enlarged text or content only in half of the textarea, an example of which is text as shown below.
The simplest way to resolve this is to remove the following function, often present in many themes functions.php file
This function is a callback for bespoke TinyMCE stylesheets. View the WordPress documentation here.