I'm using a 15" Retina MacBook Pro and am used to having Safari taking over the whole screen. BTW, I have my resolution set up for "Larger Space", which means I'm running at 1920x1200 pixels.
WordPress 3.8 fonts are just too big overall and the Admin design itself has some issues. I can't, for example, see two revisions side by side without using the horizontal scroll bar.
I realized that reducing Safari's zoom by one point makes it look much better, although the post editor gets far too small.
Please fix this, it's looking really bad. :-(