Babies not growing

Question
we have 8 babies in a floating plastic nursery. it think they are swordtail platty's or something similar. well, the mother had these babies i figure about 4 months ago and these babies have grown but at this rate it'll be a year before we can let them loose.
do fish grow in size relative to the space they are in?  are they not that big because the nursery is small?
and
whats an ok size to let them free? we have some rather aggressive fish in the tank. i cant remember exactly what they are but they look like a pickerel or a baracuda almost and are long and have a greenish tint. anyway,
should i let them out or wait it out?

Answer
Could your fish be arrowana's?  Anyway, yes, the smaller the container the quicker growth inhibiting hormones will build up.  You need to frequently replace the water with new tank water.  If they are about this <-----> big, the parents shouldn't be able to eat them.  If your other fish are bigger then Platies, then your fry may never be safe, until they are adult size.