What are some 成语 that mean something like "little dogs always bark the loudest", or "little dogs are mean"?
In a direct translation, "咬人的狗不叫", but it's not a 成语, it's a slang.
虚张声势: pretend to be strong to make others fear.
深藏不露: very talented or strong, but hiding from the others.
不露锋芒: same meaning as above.
韬光养晦: Hide one's capacity and bide one's time(Not sure it's a good match but similar with 深藏不露)
I didn't see this previously listed, and it seems to come close:
Loud thunder, little rain