To answer this
Each time you fight a battle you will see a little report screen showing the losses. Above the losses there are some letters and numbers
P:+4.50
T:+2.40
WE:+0.50
For winning that battle the winner got 4.5 Prestige, 2.4 Naval Tradition and 0.5 more War Exhaustion the loser lost 4.5 prestige, gained 2.5 Naval Tradition and gained 1.75 War Exhaustion
like in this image below
From
http://forum.paradoxplaza.com/forum/...d.php?t=450752
Range's Morocco - Casablanca AAR
So imagine this at the start of the game you have 3-5 provinces, 1 army and you fight countries with 3-5 provinces and have 1 army. You fight a long war with one or two battles per year assuming you win one and lose one you gain 2 WE per year, your monarch reduces that by up to 0.9 per year (for a ADM 9 monarch). In effect you gain 1 WE per year of war.
Now, fast forward 100 years. You are now a continent spanning empire with 200 provinces and 20 armies. This time you go to war again, against an alliance of similar size to your own country. Again each of your armies fights just as hard as that first one two battles per year one lost for 1.5 WE and one won for 0.5. This time you gain 40 WE in the first year and lose 0.9 for your monarch. There are upper limits on WE, so you won't see it go past 20 iirc. This is a bit simplified, but this is pretty much the case. You'll get more WE from attrition and other things, but since you now have 20 armies, you will gain it 20 times as fast.