I purchased two oscars, one wide tail and one dempsey and have them in a 300 gallon tank, three months back. They were all babies.

One Oscar has tripled in size, the dempsey and wide tail have doubled and the second oscar has barely grown at all.

Does this fish need a steroid shot???

as long as it remains healthy and active, i wouldn't worry. some fishes has
genetical distortion, thus grow slowly as compare with other of the same kind.
what worry me is if the bigger one grows bigger and the smaller one will fit
to the mouth of the bigger one, it will end up as "snack brother". it happen to
me in two occasion.

could it be the little one isn't getting as much food? I'm always careful to make sure my pair of juveniles each get the same amount of food (the best I can) - feeding them by hand helps:)

in my case, all of them eats like a pig but the other two remains small.
they grab enough foods that make their belly swell. however, as the other
grew up normally, they could gulp more and more as compared to those that
don't grew up. see the belly of juvenile pig

What is a widetail?