By default images are expanded to the full width of the content column, even if the original size has a smaller width. With small width images, the result are blown up, pixelated images that users have to manually resize in the editor.
The ideal behaviour should be:
Images only adapt to the full width of the column if their width is equal to or larger than the width of the column (this will ensure that most images display nicely in the same size)
Images that have a smaller width than the content column remain in their original size and should be centered by default (ensures visual harmony between differently sized images)