Are the other fish much older than Vic? And how big is the tank? Is he getting enough to eat?

It could also be that you simply haven't noticed him growing! It happens sometimes Especially if you see the fish every day!

So it's far, far, far too small. It's ridiculous how small it is for FOUR COMETS.
Each comet needs 40L minimum, so you'd better get yourself a larger tank asap. You need at least a 160L. This is most likely why Vic won't grow, there's no space for him to, and it might also be that he doesn't get enough food. Or that the food he gets isn't very nutritious. If you're feeding them flakes/pellets and only that, expect to have stunted goldfish! You need to give them variety: Blood worms, shrimp, krill, tubifex, brine shrimp... Buy some freeze-dried live foods, they're full of protein and the young goldfish need it to grow properly! You can feed them some frozen peas now and then that you thaw and pop out of their shells before feeding

The reason your fish is probably stunted and got fin rot (which can kill!) is because your tank is way way too small. That is an extremely important point. It may not be the reason your fish is small, maybe some fish are small just like humans are.

A better diet and 160L for those fish would be ideal so go do it