These severed legs with no trunk (torso and body) are all by themselves in the desert. The head of Ozymandias is broken off and lies by the statue's feet as if he has been beheaded. This is an image of destruction and loneliness. Later, we learn the following:

Round the decay
Of that colossal Wreck, boundless and bare
The lone and level sands stretch far away.

Words like "boundless," "bare," "lone," and "level" offer to our mind's eyes a vista of aloneness where once a great kingdom stood. The complete silence of this scene is also desolate. The words that Ozymandias had carved on his statue are now ironic. He calls on the "mighty" to look on his works and despair. He means for them to see his vast city, his armies, and his wealth and know that they cannot challenge him. Instead, the "mighty" now should despair because such "great" works came to nothing but desert sand. Shelley was a political radical in his time (although his convictions would not seem as radical to us), and he supported the ideals of the French Revolution. The image of Ozymandias's severed head and the "level sands," representing the forces of leveling (a word for equality), are images that warn the powerful of Shelley's day that they too will end up as nothing.