I dont think the game does a good job of reflecting the cost of raising, supplying, and keeping a large army in the field. An army shouldn't just cost influence. It should cost denars.
In real life you have to keep an open supply line for an army. The army can't just re-supply itself.
This game doesn't account for the time and money it takes to supply the army with equipment, food, horse feed, lodging, and the economic impact of a country losing the majority of its work force.....
If you put a monetary cost on raising and keeping an army in the field, it would allow small defensive kingdoms to bank money while they aren't at war.
If you made that cost smaller for countries defending than countries deep in enemy territory, since its easier for them to re-supply and find lodging, it would naturally reflect the expense and difficulty of rapid expansion.
To me, armies are making too much money from war. Historically, armies at war bankrupted countries. In Bannerlord, its the easiest way to get rich aside from exploiting the broken smithing mechanic.
Raising armies should cost denars and influence. The larger the army, the more it should cost. The deeper you are in enemy territory, the more it should cost.
Right now the game tries to make up for the expense of raising armies by artificially inflating fief income and nerfing loot income. It isn't the loot or the fiefs that's the problem. The problem is armies aren't an expense, they're a limitless supply of money.