I found out that Mongol Empire had around 100,000 soldiers on the field. Most of them were cavalries and they had great battle tactics.
Isn't that figure however a bit low to conquer and control that huge land?
The mongols weren't fighting modem armies. 100k is huge for a medieval army. Most of what the mongols conquered were steppes which weren't very highly populated to start off with. The Chinese or Persians had large populations, but their infantry army was no match for mongol horse archers.
So basically the mongols were successful because 100k cavalry armies were very formidable.
The success of the mongolian army was built on more factors:
- size: as it mentioned, an army with 100.000 people wasn't small at all. Take note that the population of Holy Roman Empire (today: Germany, Netherlands, North Italy, Austria, Czech Republic, Slovenia, Switzerland, Belgium, Luxembourg, and some parts of France) in 1200 was only 5 million!
- tactics: While Europe tried to focus on cheap infantry, and heavy cavalry, Mongolians had relatively cheap light cavalry with firing capabilities. It proved it's value before in Hunnic, Scythian, Hungarian and numerous other armies. For some reason it didn't become popular in Europe. On addition, these armies consisted skilled close combat units as well with spear and swords. Also worths to mention that Mongols mustered armies from conquered territories, so their army could get fresh supplies and manpower through advancing, lessening the burden of logistics, and practically eliminating the problem of refilling the numbers in the army.
- timing: it was the Mongols' fortune that Europe by that time sent numerous exhausting conquests to the middle east, namely the crusades, these campaigns resulted less, and it was serious waste of human resources. Mongols came just after these crusades, which gave them significant advantages. A note on this: Mongols never conquered Holy Roman Empire's main territory, but it certainly influenced the fact that literally pathetic amount of help was given to Hungary, Poland, Lithuania, Galicia. And it was Europe's luck that Ogodei Khan died in 1255 and Mongolian advance stopped.
I think the success might have had more to do with tactics than with the size of the army on the field. One of the ingenious methods Genghis Khan employed to win over the loyalty of his enemies was the order to execute them (and their families) if they would not ally with him and his forces. The majority of the nomads was reported to have realigned allegiance in order to avoid the infamous brutal onslaught of the Mongol warriors. In return, those who switched camps were rewarded a peaceful life, and promised lands and food. As enemies throughout the steppes became allies, even though the area was huge, it was more efficient to win by politics than by sheer military conquests (although that was not uncommon towards those who were unwilling to change allegiance).
Fundamentally the major Mongol success was the conquest of China, and that was due to fact China was divided and Mongols were successful in getting large amounts of assistance by allying with Chinese factions.