Keep image & post content same width?

I'm trying to make the feature image, the same size as the post content on my site. I have the image set to fill, 100% width, height auto, it has no margin or padding, and I grouped it with the text... but it still shows up smaller than the post content until you resize the window down and then back up. (see screenshots) Then it's the same size.

I know this is probably something small I'm missing but how do I make it that the image will always resize to match the post content area?