Young Frankenstein is a bitrate balloon, extremely grainy. Grayscale is not the same as actually clean dark simple scenes with little complexity. yes, grain will balloon bitrate and file size whether it be just old or whether its post production (ala 300).
I can vouch for this. RF 21 should work for these.
Oh how I love Young Frankenstein. One of my all time favorites
"Werewolf" .... "There wolf"