@maporsche,
To further elaborate, goldfish tend to keep growing to fit the size of their tank. So their 'happiness factor' directly relates to the amount of space in which they can swim around. Also the larger tank offers a greater buffer to the waste and toxic products that build up between water changes. The smaller the tank, the quicker things changing for the worst can cause a problem. I always recommend 25% weekly (chlorinated) water changes as a wise move.