Hmmm, I'm more confused now. If I disable Overlay or if I choose an alternate video output module, the picture is always "dark".
To summarise:
Overlay on, Default (DirectX) video output module, image fits on screen => "bright" picture
Overlay on, Default (DirectX) video output module, image doesn't fit on screen => "dark" picture
Overlay on, Windows GDI output module => "dark" picture
Overlay on, OpenGL output module => "dark" picture
Overlay on, DirectX 3D output module => "dark" picture
Overlay off, Any output module => "dark" picture
It sounds like maybe my complaint should be that the picture is too bright sometimes...!
Unless you advise otherwise, I'll change to DirectX 3D and manually increase the brightness.
Thanks VLC_help!