Most Blu-Ray discs will be storing the movie in digital 16:9 aspect ration but will use anamorphic presentation to display as intended and as seen in the cinema that is 'wide screen'. This should therefore result in black borders top and bottom when displayed on a standard 16:9 TV. This is called 'letter boxing'.
As a contrast old 'full frame' movies use a 4:3 aspect ratio equivalent to old CRT TVs and these would be displayed with no black borders top and bottom but instead would have black borders on the left and right sides when displayed on a 16:9 TV. This is called 'pillar boxing'.
My problem is that often but not universally VLC is displaying widescreen Blu-Ray movies with both letter boxing and pillar boxing so it has black borders on all sides and therefore wastes a significant amount of screen space. Typically affected Blu-Ray discs will show the film studio titles correctly, the Blu-Ray menu correctly but play the movie incorrectly. An example I recently came across demonstrating this is the PAL version of Watchmen - Ultimate Cut. I am attaching a screen capture below.
I also tried playing the same Blu-Ray using a different program and it was displayed properly in widescreen anamorphic presentation with just letter boxing i.e. black borders top and bottom.
This is (still) happening with VLC 3.0.8.
Screen capture can be viewed here - https://drive.google.com/file/d/1EcHvoBOxNYvRXwCfDn9mPTkAnorMXfHV/view
(I would have reported it via the VLC bug trac system but that is broken so that people cannot login to it, this was reported some time ago in these forums by myself and confirmed by others but is still broken. )