When you open a video in VLC player, the app automatically resizes its window to fit the video resolution. The exception to this case is if VLC was maximized before, in which case it will continue to open in maximized state. If you’d rather have VLC remember its window size and not resize every time to fit a video’s resolution, there’s an easy fix for it.
In VLC player, go to Tools > Preferences > Interface, and uncheck the option that says “Resize interface to video size”. Click Save and you’re done. This works regardless of whether you’re on Windows, macOS, or Linux.