Birds have a lot of fat deposits that are usually removed during cleaning. I know with chickens there can be a couple big fat pads in each body cavity. You don’t find these in supermarket chickens, but less-thoroughly prepared chickens from a friend who raises chickens or a farmer’s market usually still have these pads. Plus, the trimmed skin has a fair bit of fat as well. I imagine you can render several ounces of fat from all of the chicken trimmings.
In some European countries you might find goose in prepared foods - canned cassoulet in France for instance. Actually that’s the only example I have. But if there is a market for goose fat they will find a use for the meat.
While goose meat was not exactly rare in Hungary, it’s the fat that’s ubiquitous (along with the liver and cracklings, too.) I don’t think there’s a single grocery that doesn’t sell blocks of goose fat along with the pork lard. I’m not sure where all the meat goes, to be honest, because while most groceries or markets will have the fat, liver, and cracklings, I don’t recall seeing whole geese or meat parts all that often.