Is there a 成语 that means something like "Little dogs always bark the loudest"? What are some 成语 that mean something like "little dogs always bark the loudest", or "little dogs are mean"?
Here's some supplement to Kevman's answer.

色厉内荏："A bully is always a coward，" it literally means that looking tough outside but actually stuffed with straw. 

外强中干："outwardly strong and inwardly weak。"