Yep, the AI gets bonuses, which includes free troops. You'll want to delete three scripts: Garrison, Help Weakened Faction, and Last Stand. Open the campaign_script file using notepad or something similiar. My file path looks like this: C:\Program Files (x86)\SEGA\Medieval II Total War\mods\Third_Age_3\data\world\maps\campaign\imperial_campaign
To delete the garrison script you'll want to delete everything from:
To delete the help weakened faction and last stand script you'll want to delete everything from: