Mikhael,
I do not know if it is a proverb or just a phrase, but I think just a phrase.
The meaning of the sentence is that a person with a lot of something will always want more. If you are rich, you want to be richer. If you have many cars, you want more cars.
The meaning is about the desires and wishes of people. Humans have greed and greed will always want more. However, people say it in a funny way some times (as a joke).