Genghis Khan less recources and yet he conquered almost as much land as Caesar. He alone forced the entire Mongolian nation to come together into one force. He used his ruthlessness to his advantage, unlike Julius Caesar. Julius Caesar was the best president, but Genghis Khan was a much better leader.
Genghis Khan was a great military strategist because he inspired rival tribes to combine as one. He knew that by devastating the people he attacked that they would never be able to cause an uprising against him. Julius Caeser could not even find his way out of a paper sack.
The main problem of mongols were that they can only offend, but were not the best defenders. Only after a small time when the mongols were formed, they simply lost all of their lands.
Caesar, on the other hand, kept his empire. For another 500 years, rome just got better and better. Even now, caesar leaves his legacy.
Also, Italy in general has a more powerful millitary then mongolia.
Caesar had to win elections in Ancient Rome, but Genghis Khan was just a tribal chieftain. Caesar has left us several books about his strategies, but Genghis Khan couldn't even read. Hundreds of millions of people speak Romance languages today because of Caesar's conquests. Not many people speak the Mongolian language though.