I see you are building yourself a neat Constantinople army there:thumbsup2 Are you going to spread those siphonatore's around the map?
AFAIK the AI does have a population bonus and by memory, Constantinople is still the biggest city in an Early Era game at the start, so its still no surprise that you were able to build it up the Huge status so quickly - Plus if you are able to build all of the School line buildings, your governers get some awsome economic + pop.growth traits which come in very handy.
I wouldn't worry too much about the Mongols to begin with as it looks like you still have a nice buffer between yourself and them. When the time does come, just make sure you have plenty of missle cav in your stacks to counter them.
Nice work so far too by the way.