Can you check if "Don't resize video in full screen" is selected?
(under rightclick > Options > Preferences > Video > [General] )
If so: deselect it.
Further: Is "resize OSD and subtitle font automatically" selected?
Under "Preferences > Subtitles" maybe check "Draw subtitles to overlay surface"
(P.S.: For wide screen the option "Cut top and bottom in full screen mode" may be of interest.)