Image size multiple images in posts default full width.

Large images are now extruded from a medium size. When i look at the input (firebug) of the images in my (portfolio-)posts (single view) I can see the medium size is used, that size seems to be the default in the option list of the image as well. The images are shown at 70% percent of the screen width and therefore i would like it to use full size as default, at least for large screensizes. Is there a way to make full size default at upload? Isn’t wordpress capable of choosing the proper image size depending on the to be filled width? Or can my theme wire bein the way?

To be clear i am adding multiple images in the post itself, Not as a gallery, and Not as a featured image.