If you get various Oscars all at the same time from the same batch then the chances are they will grow at a uniform rate and be of a similar size when they are adults. However, this isn't always necessarily the case, you just may find that one Oscar grows bigger than the other even though it may be eating just as much, as to why? I have no idea. It may be like many creatures in the natural world, you sometimes get very strong healthy animals that grow, and then others that may be a little bit weaker. Maybe someone else can shed some light on this, it's not actually something that's ever really bothered me, although I have had a couple of Oscar that have not quite grown as big as their siblings.