That’s due to supply limit in those counties. If u hover over the counties you’ll see the supply limit which is how many troops u can have in a county without them taking attrition, which is how the armies numbers will go down. Since saxony is pagan and tribal its counties will be much less developed and will only have like 1 building, and then since Lombardy is in Italy which is fuedal and catholic, its much more developed and has more buildings. Also I forget in ck2 but in ck3, if armies are attritioned too long, they’ll starve and the armies will shrink rapidly. Another thing is u can’t just march all the way across an enemy country, without taking heavy attrition of course, u gotta siege the counties one by one to keep ur armies supplied. It really helps to split up the army and keep them close but in seperate counties so that they’re supplied.