The two red lines at either side of the below image delineate the boundary of the area containing the contents of the post. Observe that the right side of the nested image extends beyond the red line.
Here's a high-contrast version, in case your eyes aren't sharp enough to spot that (mine nearly aren't):
That optical difficulty would not be replicated with an image that was not substantially white (as no doubt many images used on SE are).
Note that the offending images are within a blockquote in a list item - excessive nesting could be the cause.