I have been having this issue for a long time and I am not able to correct the problem. When I open a window in IE 11, there is not a normal full screen. The window opens in a smaller window. Even though I click on full screen icon, if I exit IE and later go back in I get the same issue. It also happens when I open IE from inside a program.
Internet Explorer will remember your window settings when it was last closed. If you exit it in full screen mode, it should open in full screen mode the next time you open it. But sometimes when you open a link, it may open in a smaller screen. In researching the issue, this may have happened after you had some malware or virus on your computer. (Quick bonus tip, if you want to go to Full Screen quickly, hit the F11 key.)
If this is happening, I found a few solutions. First, you can try this. Open one window in IE. Then, right click on a link and choose Open in New Window. Once that opens, close your original window. Resize the current window to the size you’d like your windows to open by dragging the corners. In this case, don’t use the Maximize button. Then close this window. Hopefully, when you reopen the window, it will be at that same size.
Another option is to open IE, drag the window to full screen, and then hold CTRL while clicking on File. (If the File menu is not there, press ALT before holding CTRL.) Then click on close, and reopen to see if it keeps the window the size you wanted it.
If none of these work, you may want to try resetting IE. You don’t have to Delete your personal settings. in this case. Remember,though, this will remove all tool bars and changes you’ve made to security settings, so you may have to make a few adjustments in the next few times using IE.