This I think is THE biggest military problems the game has.  Once the human gets into a steamrolling mode, the near lack of invader population loss during a successful invasion allows the human to just keep on rolling.  

The AI is much more conservative in this manner when it could be performing invasion steam rolling.  Plus, the AI will perform invasions which lose, rather than amassing enough troops to ensure 100% victory all the time.  More important, even though you have 100% chance of success, the more troops you have, the less you will lose upon a vcitory, which is certainly a concept the AI doesnt understand, but the human surely does.

By properly correcting this, it will make offenses slow down a bit, as population will need to be regrown and thus give advantage to the defending empire.  It will indeed make the game more challenging.