Some of the breeds naturally have fat bellies, especially the fantails. Many goldfish are now crossbred so unless you got them from a serious breeder, they will probably a bit of everything.
It is getting a bit late in the season for goldfish to breed so it is unlikely that they are full of eggs. If they are they will either lay them if there are males present, or re-absorb them. Nothing to worry about.
If they are that fat that the scales on the side stick out that means a serious ailment, probably infectious. In that case you need to catch one and take it to a fish shop for a diagnosis and possible remedy.