In article ink.net,
says...
I've seen some big (4 - 5 inches of body length, 4 inches height, and 2 - 3
inches width) fantails and other goldfish varieties at LFS. All of them
were slower than molasses. Is this the future look of my fantails? Big,
fat-looking, slow-pokes? Or are these fish too-fat and unhealthy?


Um....we fancy goldfish fans like to think of them as voluptuous and
unhurried. If you want tweaker-fish (that is, fish that dart around
like they shoot meth) you may want to forego goldfish.

To quote that great philosopher/blues composer (and goldfish keeper?)
Willie Dixon:

Some folk built like this, some folk built like that
But the way I'm built, you shouldn't call me fat
Because I'm built for comfort, I ain't built for speed
But I got everything all the good girls need.

(Made famous by Howlin' Wolf, reportedly 300+ lbs, size 16 shoes(!))

Of course, it's all relative. The ones you've seen may be
in too small an environment -- can't move freely, so why move at all?
They could be in cold water, which slows them down and saves on food.
Or they could be sick or otherwise stressed.

Gunther