Habits Of Space
If a Goldfish is used to a certain area in which to move and live, does it stay within that space and not recognise that it has a larger area in which to move and stay confined to the smaller area.
This is an interesting question, which I haven't tested.
Goldfish are definitely creatures of habit, which is why we can train them to feed out of our hands by feeding them at the same time and location each day.
If the area a Goldfish was living in suddenly became much larger, the Goldfish would be aware of the change.
If Goldfish are introduced into new surroundings, they tend to hide for a period before exploring their new environment.
If Goldfish are able to get enough food in a small area of a larger habitat, I suspect they would stay in the same location. Why take unnecessary risks?
On the other hand, Goldfish are browsers, continuously looking for food. This behaviour would suggest that they would need to spread their search for food as far as is necessary to meet their insatiable appetites.
Goldfish are also individuals. Some are quite fearless while others are annoyingly timid. Watching my own fish feeding, some will pick around the same open area of the aquarium while others will seek out hard to get at nooks and crannies.