What assumptions do you make to come to these conclusions? What are the criteria you use to separate them? Is it okay to raise an Oscar in a 2.5 gallon tank? Why or why not?
Now you're just dramatizing. Oscars would out grow a 2.5 and that is the equivalent of giving a lab a kennel half it's size. If it is physically incapable of surviving in there than that's a different topic. The goldfish has a long time to grow and you're assuming nothing would change on the owners behalf when it grows.
Tell me why you say a goldfish is in pain, why it doesn't like the situation.
The goldfish doesn't think it is cruel, he is just trying to survive to pass on its genes like all animals. You could say lakes that dwindle in the dry season in Africa is cruel. But do animals think of it that way? No. It's human emotions that personify everything that they see fit.
Not allowing an animal to reproduce could be considered just as cruel as its environment it's in. Since, in nature, that's the main goal of life.