I don't know what other feed their piranha or how often to feed them.
But for me you can over feed your piranha easy. piranha never stop eating, and it's not good for them to eat alot and kill them self.
let say you feed them alot once a day feeding time...rather do that I would feed them 2-3 time a day with small portion. It give them time to relax. don't over kill them with food, food are not the only reason for piranha to grow.
you can feed alot and over feed them, but if you don't have good water, and tank size piranha will not grow as fast as you want.
If you want your piranha to grow fast here is some tips from me.
Tank size, as big as you can for them. the bigger the better.
feed them small portion, 2-3 time a day, or once a day for adult.
keep water clean, do water once a week 20-40% plus vacuum the gravel.
lots of filtration, piranha are messy eater and produce a lot of waste.
all these will help your piranha grow faster, not just food and over kill them wits it.