The images inside blog posts and inside portfolio projects are actually the same size if you use images of the same aspect ratio. For instance, see the two images standing side by side in this post and the images standing side by side in this project.
That being said, the cover image in blog posts may often be a bit wider than the images in portfolio projects. To make them the same, you can add the first image into a blog post the same way you add it to a portfolio project (inserting it via Gutenberg). In this case, you’d need to hide the blog featured image from the blog post page, which can be done with the following line of CSS: