Minions are small to show that Gru is surrounded by incompetence
Minions are known to be stupid, disorganized, and even naive, and these are traits that no serious criminal mastermind wants his subordinates to possess. Hence, the Minions are perfect characters to showcase Gru’s incompetency as a supervillain. He isn’t as professional as he thinks he is.
It’s quite clear from the onset that Gru looks up to his ancestors. This makes sense as he came from a long line of supervillains. This may explain why he tries hard to make up for his lack of success by using his army of Minions to cover his incompetence.