At simple economy draft size depends on MaxMobilized column. For example, with a MaxMobilized value of 300 you get a draft size of 60. And with a MaxMobilized value of 150 you get a draft size of 30.
But at advanced economy things are not the same. France has a draft size of 20. Britain 11. Sweden 3. Prussia 7. Austria 19.
I think I have got it. It depends on the population you have. The more population the higher draft size. So, if you conquer some provinces the draft size grows.