武大郎 is a legendary short man. He runs a shop; and all of his employees are shorter than him. It characterizes an orgnizational phenomenon. I wonder if there is an English saying that means the same thing.
武大郎 is a single personal sesame cake maker and vender, he had no shop, and he had no employees. He died of murder by his wife, and revenged by his younger brother 武松， this is a famous story written in book 水浒. And 武大郎 is famous for his characteristic of short. And 武大郎开店 in the allegorical saying has the explanation ( you can see it as answer for this riddle) of 长子免进（or 高我者不用）, which means He will not hire who is taller than him. So this is a metaphore of describe someone being jealous of man of talent.