He's not very big, he's sitting at just the right spot of that cup, and it makes him look much bigger than he is.
That's the cup that the petstore sent him home in, and I think I like the bags better, if you hit a bump, it's not going to hurt him to hit the side of a bag, on a plastic container, I was afraid it might injure him on the way home!! Plus, that cup is only about 6 inches wide, and he had plenty of space to swim around in while we were in the car.
I've read a dozen sites, and they always say x is the MINIMUM size you'd want to go, but they always love bigger and better spaces.
for example, piranah-info.com said 8-10 gallons for a juvenile is enough space, and I have mine in a ten, it doesn't mean it's his permanent home, b/c I know he'll get bigger, and he'll need some buddies, but, until I move my PET goldfish somewhere else, he'll have to hangout in there.