When you hover over a person in their dynastic tree, you will be able to get an idea of how many troops they can summon. There are several numbers, but I can't figure out which I should look at.
Now, what I think, is that the total levy is how many they can theoretically raise, when their garrison and levy is full. The current is the number of troops they can currently raise (maybe they've just been at war, or some territories are recently conquered).
The relative strength I don't know. I thought it had to do with how many of those troops are currently unraised, however in this example the good King actually has a standing army getting their knickers handed to them by some helpful rebels.
Am I far off?