:help fullscreen
...
Note: Setting 'fullscreen' usually changes the size of the Vim
control. However, for technical reasons, 'lines' and 'columns'
will currently only be updated when Vim runs its event loop. As
a consequence, if you set 'fullscreen' and 'lines' or 'columns'
in a Vim script file, you should always set 'fullscreen' after
setting 'lines' and 'columns', else 'lines' and 'columns' will
be overwritten with the values 'fullscreen' sets after the
script has been executed and the event loop is ran again.
Also see:
:help fuoptions