the large picture size makes the entire thread very wide so that it runs way off to the right of the screen - matching the width of your posted pics.
Hmmm ... I'm not seing this with the new forum software that was recently installed. It used to be that way IIRC. But now, I just see side-side scrollbars on the images, not the entire thread. Thread width is normal (I'm using Firefox as my browser, but I don't think that matters).