Uploading or using an image from the Media gallery comes by default as centre aligned. When published the image takes the full width of the container it's in. If a different image size is selected, the image continues to be full-width, but changes the resolution based on the selected image size.
The way to have an image follow a size change requires the user to set the alignment, even though it comes centred by default. Once the alignment is set this stops the image from taking full width and is possible to set the size.