Are you asking about this ?
If you asking about what happens if you want to level with 100 heroes / troops for example, then the food/iron cost is only constant only for 10 heroes / troops, and after that it increases to the actual number on the proper level, and it is constant again for 10 heroes / troops.
So the food / iron cost is the exactly for level with 100 heroes / troop compared to if you do 10 leveling with 10 heroes / tropps at a time.