Cannot Display Medium Sized Image

My wordpress website won’t let me select my own display image size.

I want to display an image in Medium size. But it always stretches the image out to full width of the blog post.

Please see ‘true manifesting from the soul’ image in the middle of the blog post.

Blog post: https://millennial-grind.com/how-to-get-your-ex-back-with-law-of-attraction

I have another website on the neve theme and I can display medium sized image with no issue: https://cookingwithkenneth.com/best-oven-roasted-potatoes/

Please help me identify what is causing this issue.

The page I need help with: [log in to see the link]