the little Dat i got seems to be pushed around by fish it same size. but although he's new in the tank...and there are other fish much larger. i been reading that most Dats can be aggressive tho.
Just give him time to chill and get used to the tank.
Had the same problem when I introduced a 5'' American tiger in a tank with a breeding pair of jags, and other large fish. A few weeks later he started bullying the other fish lol.
Size does play a role as larger dats become more aggressive. I've seen my most passive ITs turn into animals and beat up everything. Just gove ur shy dats some time. Mine are all the first to eat! Lol