Original posted deleted by me. Different day, different mood. If it was seen, it was aimed at both the OP and Jim, and was uncalled for on my part. Also explains Jims' post.
For the OP. The only thing I can think of is, you did or something else happened and your browser has shrunk. Sounds like maybe browser has gotten collapsed and needs to be restretched. I have seen this happen before, unsure what actually causes it, but this may be your fix.
Just tried it with Chrome, you can collapse the window and full screen still takes it to full monitor size. But I did use the menu to select full screen. Has nothing to do with zoom, as that is only within the window. If monitor resolution has been changed it can affect window size.
Left click on the browser window and hold, move the browser window up to the upper left corner or your screen. Release left click, go to the lower right corner of window, When at the corner your mouse cursor will be replaced by arrows. When that happens click and hold left mouse, drag that down to the lower right corner of your screen and release.
When you close down the browser, the operating system should remember that window size.