Well, see it in another light. They killed 450 Lorenvolk, but lost 2720 against them. That looks about right to me on paper. 

Your militia let you down, but it serves as a lesson. Marching them against professionals has draw backs, their numbers are their true strength. 

If you want a bonus tip: The Lorenvolk are supreme warriors, and whilst it might be tempting to split them, perhaps marshalling them into one main battlegroup would serve you well early on. At the moment, as with the Isarimer, human numerical superiority is taking a chunk out of the race bonuses. The more men you field, the less advantage the humans have. 

i.e 10,000 Lorenvolk vs 10,000 humans would be a giblet fest.

Meanwhile the militia could either dog pile the army, to act as a shield in battle, or be kept back to make sure your provinces are behaving themselves.

Also, feel free to PM me before you go invading places, and I'll tell you their force strength. Minor nations pick a random number between two set points (I sometimes will lower it if the country is tiny).