The power wielded by Ozymandias comes through in the poem from specific word choices as well as from the overall image created.
"The sneer of cold command" on the face of the statue implies great power. The king was able to deliver his orders without relying on the goodwill his people felt for him. He regarded them with contempt; he treated them without warmth. Yet he feared no rebellion from his cruelty because of his great power.
The line that probably best conveys the king's power is this one: "The hand that mocked them, and the heart that fed." Again, this line tells the reader that the king operated as a vicious predator over his subjects. His hand mocked them, holding them in derision, not caring for their needs and knowing they could do nothing to threaten his reign. His heart fed upon them, as if they existed only to meet his desires. They were completely under his control. When he raised a hand, that determined their fate. When an impulse charged his heart, he fulfilled it at their expense.
Finally, the fact that the traveller can glean so much information about the ancient king's power merely from a fallen statue drives home the immensity of his authority. After all, "those passions ... which yet survive" are only "stamped on these lifeless things," yet they still come down through the ages, proclaiming how ruthlessly and unopposed the great king reigned in his day. Although no one could stand up to him while he lived, he succumbed to the rule of time, and now only the broken statue attests to the power he once wielded over his victims.