Elbows have a reputation for being a dirty, but highly effective technique used to end a fight. Inherently, they're powerful strikes, but a common misconception in the MMA and mauy thai communities equate the size of the muscle to the power of the elbow. On the contrary, muscle mass has little to do with contributing to the overall power behind the elbow strike.
Instead of throwing your elbows as if you would a ball, concentrating you power in the upper arms and shoulders, think of the throwing the elbow strike with your entire body. Think rotational force, so instead of relying on arm strength alone, you're levering the rotation of your entire body, from the pivot on your feet, to the drive of your hips, and your entire upper body. The key is to learn to throw your muay thai elbow strikes with your entire body moving as one. Take a look at the little instructional video I did on the basic mechanics of the muay thai round elbow for your own training purposes.