How to Send More Troops
Battle size seems to be influenced by the terrain and infrastructure of the area.
This can be why having troops with better quality can be useful, since they will do higher damage quality similar sized armies.
Since eventually they can run out of troops to support the divisions in the field.
Note, if 6 divisions is their entire counties army and they are coastal. Consider doing a naval invasion somewhere on their coast.
Since it can open up another front and force them to split their forces, or abandon a front.