I cant comment on 4th ed as i dont have it,but ive worked with 3rd edition mass combat.
There are a lot of factors taken into account.
for example to your comparision. orcs would have a Troop strength. this is determined by the skill level of the troops and how many battles they have fought in,as well as just a racial number.
giants for instance have a higher TS (troop strength) and so for example a elite giant warrior going against 5 -10 green orks would wipe the floor with them.
like i said factors come into play! 1 is troop size (in your case this would be the horde magnitude! i have played deathwatch so i know what you are on about.)
If a horde is larger then its opposing force it gets given a bonus +. if its a racial enemy ,thats another +1,holding a position say a city is +8 and you add all these factors up giving you an overall Troop Strength to your army and theres.
heroes of 100-150pts carry bonuses too.as you can use them to do special events. like make a play to assassinate the enemy commander. and you allocate how much risk i think it is to the task.
you work out all these events, and their outcomes,troops vs troops determines a level of loss of troops and hence a victor.
Number of battles fought etc give you an over morale too.as well as your commanders leadership,strategy and something else ive gone mental blank on.
i have it and can try and help you work some out at least to how it was in 3rd. whether then you purchase 4th is up too you.