the more troops you have the more it costs them to break you. 1m troops takes 40k oil
if troops are 140 thats 140m initial cost plus oil being at 350 is expensive af so 14m there for the first hit slowly going down as they send less but if they are oversending units they will lose their troops faster and will need to spend more money to keep them up with the break
if you run low troops you are an easy cheap kill, the more you have the more it costs and the less they can do regardless of how many turns they have.