Power chords are typically played with the root note (in this case, the E string) and the fifth note (the A string). When you pluck the E string, the vibrations of that note will also cause the A string to vibrate at its resonant frequency. This sympathetic vibration will cause the E string to vibrate more than it would if it were played alone.
The amount of vibration of the E string will also be affected by the thickness and tension of the strings. Thicker strings vibrate at a lower frequency, while thinner strings vibrate at a higher frequency. Higher tensioned strings also vibrate at a higher frequency than lower tensioned strings.
Additionally, the type of guitar and the materials used in its construction can also affect how much the strings vibrate.