For whatever reason, Adobe Acrobat will not properly repaint the screen when resizing internal windows. It 'sticks' the image at the previous window size.
The only fix I've found so far is to show or hide a secondary pane, such as the bookmarks. This forces Acrobat to repaint the screen to the new scaling.
This problem may be unique to my FirePro 8800 triple monitor setup, or it might be a W7 thing, I'm not sure. Acrobat is not the only program having this problem, but it is by far the worst.
[Addition]: You can quickly force a refresh with F4.