This makes me think about the attrition and reinforcements in Europa Universalis. I think the idea is good overall, wih 2 comments. It should affect only large armies, with a gradual increase (maybe starting to have an effect over 100 troops?). And then this mechanic should not add hard limits on what you want to do. For example you should still be able to attack the enemy's capital in mid-teritory if you gathered enough resources and prepared for the campaign. In such case it would be a nice aspect you must keep account in the late game and it won't give you headaches.