If it looks fat because its scales are sticking out like a rasp or file the fish is dying and has a disease called Dropsy. If it is not swimming right that is usually caused by the same kind of things but the problem this time is with its swim bladder. These problems are usually caused by bad water conditions (Insufficent water changes, overcrowding, too small a tank, poorly set up filter etc). MY advice is do a water change ASAP.
If the goldfish is swimming in water, it is alive.
Swim bladder disease? http://www.straightdope.com/columns/read/1688/if-a-goldfish-is-swimming-on-its-side-is-this-a-bad-sign
Yes
it stops swimming
No!
Gravity
Their are many varieties of goldfish, however goldfish with fat or potbellied stomachs are classified into the "Egg Bodied" goldfish category. Egg Bodied goldfish are their own distinct group of fancy goldfish which all have an unusually large abdomen. The most popular goldfish in this group are the Orandas, Ranchus, lionheads, and the pearl scale/Hama Nishiki.
Depending on the size of the goldfish right now, it may take as little as 1 year to as long as 3 years for it to reach adult size.
if it is not swimming and it lays on its side then it is dead.
You can lose leg fat by swimming because the resistance of the water causes you to burn fat in your legs. Swimming is a good way to develop muscle and lose weight.
swimming raises your heart rate with will burn fat everywhere in your body, not just your thighs, bike riding and swimming would help lose fat
They will be really fat.