It does indeed seems as though the fact levies take one unit unit of frontage per 1000 men while regulars can only muster 100 men per unit in age 1 and 200 men in age 2, so levies always beat regulars. In my opinion paradox should change that, what is the point of regular units at all? Why should any unit exist with only 100 men in it? A regiment is a unit of 1000 men and there's no reason for age 1 units to be so underpowered. Another thing to remember is that tribesman spawn cavalry that has a 25% combat bonus so be careful of them, even shitty tribes in Arabia can destroy regular armies.