The Yud is a directional letter moving from the left to the right. The direction begins with man, and flows towards Hashem. We say the Yud is a power letter because the Gematria of the word Yud is exactly double the Gematria of the letter Yud. The word YUD is twenty and the letter Yud is ten. Man was created to offer everything in his possession to Hashem.
Power is one of the most difficult areas within man's possession to relinquish.
The Yud is like a sports trainer who challenges his players to train properly. In the same way the smallest letter challenges man's ego to come into the gym, to workout, to train, to discipline and to channel the power. That is why the Yud is so small. It challenges man to reduce, to relinquish that power. The Yud is like the word Coh Ach, meaning power.