Just a note: for any of a number of political reasons, leaders often find it necessary to deploy troops to defend all their territory (or at least populated territory) even when generals tell them a better and less risky defensive line lies in the interior.
Be glad there isn't a rule that "encourages" human players to garrison all their frontier provinces (dissent penalty, lack of confidence in the government, weaker party control, increased risk of coup, seen as weak by foreign powers, etc).
This is something that I miss in the game. Some forced sentiment of leaving a garrison in heavily populated areas or else it will impact your countries unity or political stability.
I usually try and play as realistic as I can and leave Garrisons in all my major areas (where I have industry). The larger the industrial area the more brigades I put there. So provinces with ten industries get a full Garrison division while some small industrial areas only get a single brigade.