Subject says it all. How much of a resource it takes to tops off a feeder should depend on the resource used. (Much like happens with the Still, where 1 potato is more units than 1 corn which in turn is more units than 1 grain.)
A pig doesn't really know the difference between expensive food and cheap food - to a pig, all food is very much equal. I think that's kind of reflected in the piggy feeder's attitude too.
+1 Omnika. I was all set to agree with San Serif, but then I read Aviatrix's comment, and then I agreed with that one! Pigs don't care, as long as it's food!
When did a piggie care if it was a awesome stew or grain? Piggies don't care what kind of food it is. As long as it's food, they're happy. A grain and an Awesome Stew feels the same to them. And why not just put grain in piggie feeders? It's so easy to obtain, costing only eight squeezes and sixteen energy with Animal Kinship seven! And Awesome Stew costs a lot to make, so why put it in there anyway? If you don't like how you spam Enter too fast and accidentally put in your Awesome Stews, make two Abbasid Ribs.