fat goldfinches

I'm beginning to wonder whether perhaps I'm doing Nature a disservice by feeding her goldfinches. Some of the regulars at the feeder have become rather tubby. I'm looking at one now who can't even dangle upside down anymore to peck his food, because he's gotten all oblong and wobbly. Poor fellow. He tried it just now and fell right off. And when he stands on his little perch his tummy hangs over his feet. I don't think goldfinch tummies are supposed to do that.