The widget: Featured Categories Widget uses thumbnails

The widget: Featured Categories Widget uses thumbnails to showcase the articles. Therefore,
looking at the function inside widgets-functions.php, the script calls:

<?php wp_custom_post_thumbnail($the_post_id=$the_post_ids, $with_wrap='', $wrap_w='', $wrap_h='', $title=get_the_title(),$fetch_size='thumbnail',$fetch_w='', $fetch_h='',$alt_class=''); ?>

which is a theme function using fetch_size="thumbnail".

However, looking at the wp-content/uploads files, every time I upload a photo, multiple sizes get created.

Ideally, the thumbnail is square, I have set this up in the Settings - Media section.

A square image is created for 300x300 and 200x200. However, other sizes get created like 300x150 ...

I'd like the square thumbnail to be used but it uses that for some and the 300x150 for others.

Any ideas?