Yeah that's the point, you can't catch an ennemy with an army so the smaller and faster party catch them for you, I have seen weird behavior tho were some guy suddenly attacks me with 1/3 or less troops then me I assume because his allies were in a nearby castle and get destroyed but even a troop of like 6 steppes bandits attaqued my 100 men army for no reason (I wasn't even chasing them they are way too fast)