There is a maximum number of starting points for defenders and attackers in a battle. There are more starting points for defenders in a city battle, but if you have a lot of militia defenders you can still exceed that if your city garrison is full or overfull (such as when you have nine defenders and then a hero gets defeated and goes to the nearest city - this can exceed the normal 9 unit maximum for garrison). So, when you are attacking or defending, if you have more units than there are starting positions for your side on the tactical map, you simply won't fit. Stardock would have to create bigger tactical maps (probably only city battle maps - not sure if this happens with any other maps) with more room for attackers and defenders to completely solve this problem, which I hope they do.
