SALVATOR: Vypada to tak, Gemini mi pri hledani detailu jeste nabizi :
That detail about the husband only eating fat is a crucial clue! It strongly points to a very well-known and beloved Czechoslovakian animated short film.
The film you're likely thinking of is "O tlustém dědečkovi" (About the Fat Grandfather).
It's a classic stop-motion animation from 1970, directed by Ladislav Čapek, and it perfectly matches your description of an old couple where the grandfather has an insatiable appetite for fatty foods. The film is quite charming and humorous, depicting the grandmother's struggles to satisfy his unusual diet.
This film is sometimes referred to informally as "Fatty's Hole" or similar, but the official title is "O tlustém dědečkovi."
You should be able to find it by searching for "O tlustém dědečkovi" on video platforms like YouTube.